According to Wikipedia, the US radar station SCR-270 picked up the Japanese approach and reported it, but this report was ignored as the duty officer, who believed the sighting to be American bombers coming from California. It also states that SCR-270 was still in training mode. So far, everything is clear in this specific case but, looking at the bigger picture, I can't find a reason or reasons why. The article SCR-270 goes on to state:

Despite the high level attention and the excellence of the school in training on the use of the SCR-270 and its integration and coordination with fighter intercepts, the army did not follow through on supporting the junior officers who were trained at this session

Further, Wikipedia states that:

Except in rare cases, there was little interest in assisting or even cooperating with the goal of setting up the air defense system.

The British had already demonstrated the value of radar, and shared both that knowledge and the specific technology they were using with the US.

Aside from the tendency of some people to ignore things they don't understand (which appears to have been the case with some army officers, though this may be hard to prove), are there any specific reasons why this 'high level attention' was not properly followed up on?

  • 5
    In general, armies don't like new ways of doing things. It's commonly said that they're always preparing to fight the last war, but in my experience it goes well beyond that. They're still teaching things like close-order drill that have been obsolete at least since the American revolutionary forces ambushed the Redcoats at Lexington and Concord.
    – jamesqf
    Commented Sep 10, 2017 at 17:59
  • @jamesqf: I thought close-order drill was no longer taught for use in combat, but rather a) to instill discipline and b) to look impressive during parades?
    – Vikki
    Commented Nov 21, 2018 at 18:03
  • @Sean: I am not privy to the thought processes of the higher brass. From my own experience, it did not work to instill discipline. As for looking impressive in parades, despite the opinion of the current Commander in Chief, that is NOT the purpose of a military force.
    – jamesqf
    Commented Nov 22, 2018 at 3:38
  • @jamesqf: Not the primary purpose, no... but definitely a secondary purpose, in that impressive displays of one's military might a) presumably encourage at least a few more people to enlist who otherwise wouldn't and b) would tend to make potential enemies more likely to think twice about becoming actual enemies. ("Might" is here being used as a noun.)
    – Vikki
    Commented Nov 22, 2018 at 4:00
  • @Sean: While I agree about the possible advantages of showing impressive military might, I don't think a parade does that. IIRC Saddam Hussein used to hold quite impressive military parades, and look how well that worked.
    – jamesqf
    Commented Nov 23, 2018 at 16:45

4 Answers 4


I'm going to draw heavily on United States Army in World War II The Western Hemisphere - Guarding the United States and Its Outposts. I would argue the US Army was very quick to adopt radar, just a few years, but events unfolded faster than they could fully integrate their lessons.

Rapid Modernization and Expansion

In 1941, the US military was still undergoing a rapid modernization and expansion started just a few years before. Everything was being upgraded. New equipment was just being delivered. Everyone was having to very rapidly adapt to the new technology and tactics of WWII. Not everyone adapted well. This report on the state of the Panama Canal defenses lays it all out.

The recurrent crises in Europe during 1938 made the weak spots in the defenses of the Canal seem glaring indeed. With respect to antiaircraft, coast artillery, and air forces, the situation was particularly acute. The actual strength of the two coast artillery regiments was inadequate for the proper manning of the seacoast defenses, and as a result the infantry troops had to be given double assignments and dual training. The existing system of fixed antiaircraft batteries lacked, it was believed, sufficient depth and mobility to offer an effective defense against high speed, high altitude bombers. The air force was equipped with obsolete planes. France Field had been outgrown for some time, and room for expansion was lacking. The main runway of Albrook Field was still under construction. Moreover, it had become increasingly clear that by the time hostile planes came within range of the existing Army defenses it would be too late to prevent them from delivering an attack on the Canal. Effective air interception would require long-range patrols, radar installations, and a screen of outlying bases. Not one of these requirements was available.

Source: Chapter XII - Forging the Defenses of the Canal

General Short started his military career in 1902 with horses, runners, and bolt-action rifles. The anemic interwar US military budget meant his experience would stay that way for far longer than other militaries. Now he's overseeing machine guns, radio, motor transport, tanks, aircraft, and now radar. It must have been dizzying to keep up with and assimilate all the new technology into his thinking. This was the pattern for US senior officers.

Radar was brand new. Its potential to be a decisive factor in an air battle was only just demonstrated in the summer of 1940 during the Battle of Britain. The US had a little over a year to react.

The first sets of SCR-271, the air early-warning radar, were installed at the end of 1940 in Panama to get their first real use in the field. The junior officers operating these sets went to a brand new air defense school in April 1941 for training and to be taught the lessons from Britain, but the senior officers did not. As a result, radar was poorly understood at the command level and poorly integrated into air defense.


Pearl Harbor was primarily concerned with sabotage, yet still put the brand new radar on alert.

General Short's action had been to order an Alert No. 1, as defined in a new Standing Operating Procedure dated 5 November 1941. This alert assumed increased danger of sabotage and internal unrest, but no threat from without. Under it the Army, in General Short's words, "put out a lot of additional guards and checked on everything," and for the two infantry divisions this meant keeping thirty officers and 1,012 enlisted men on guard and patrol duty. The Hawaiian Air Force was ordered to concentrate planes so that they could be guarded more easily, and these orders were as easily executed since that was the usual practice at Hickam and Wheeler Fields anyway. The only deviation from procedures prescribed under Alert No. 1 was an order directing the operation of the new Army radar machines between four and seven each morning--the most likely period for a carrier strike, according to previous studies.

Source: Chapter VII - The Pearl Harbor Attack

Keep in mind that at this time, carriers were still very new and mostly untested. The British had demonstrated the actual power of aircraft against ships in harbor at the Battle Of Taranto and that lesson was still being absorbed. There were so many other targets between the Japanese home islands and Hawaii, and the war hadn't even started yet, that an unprecedented massed carrier strike at Pearl seemed a very remote possibility.

When first questioned, General Short said he ordered Alert No. 1 for three reasons: first, he thought there was a "strong possibility" of sabotage, and he feared sabotage more than anything else; second, he had no information about any danger of external attack; and third, either No. 2 or No. 3 would interfere very seriously with training--"it was impossible to do any orderly training with them on."


During the conference on the 27th Admiral Kimmel turned to his War Plans officer, Capt. Charles H. McMorris, and asked specifically what the chances of a surprise raid on Oahu were, and the answer was "none." No one of the other Navy officials present challenged this judgment, and General Short saw no reason to question it. Both he and his naval colleagues were also heavily influenced by the knowledge that Japan could not attack Oahu with land-based planes, and by the continuing assumption that the Japanese would not risk a carrier strike as long as the bulk of the Pacific Fleet was in or west of Hawaiian waters.

Source: Chapter VII - The Pearl Harbor Attack

Army/Navy I-Cannot-Put-Enough-Quotes-Around "Coordination"

The US Army oversaw the defense of Hawaii, but Pearl Harbor is a US Navy station. This lack of a unified defense for Hawaii meant two split command chains who would communicate and coordinate with each other only slowly and begrudgingly. This is an unfortunate reoccurring theme throughout military history.

The Navy was responsible for seaward defense of the island. The Army was responsible for the land and air (the air force being the Army Air Force).

Whereas the directive of General MacArthur to undertake reconnaissance was a sensible one, since that was at least partially his responsibility, it was not applicable to General Short's situation, for seaward reconnaissance to any meaningful distance was recognized in Hawaii as strictly the Navy's business.

Source: Chapter VII - The Pearl Harbor Attack

The Army operated the radar sets, but the Navy knew what was going on at sea. When a midget submarine was spotted hours before the Pearl Harbor attack by the Navy, nobody thought to tell the Army.

On Oahu the military forces did obtain other warnings of impending action. More than four hours before the air attack began, one of the midget submarines was sighted less than two miles outside the Pearl Harbor entrance buoy, and either this submarine or another like it was sunk near the harbor entrance at a quarter to seven. According to standing orders, the presence of any unidentified submarine in restricted waters was to be considered a warning of imminent attack on a larger scale; but the Navy was still in process of checking the authenticity of reports of these submarine actions when the big attack came. No one thought to tell the Army about them.

Source: Chapter VII - The Pearl Harbor Attack

The rapid introduction of new technology without training the senior officers responsible for its integration, the Army's focus on sabotage, and the lack of a unified command meant that when the strike was detected by radar there was no urgency to report it.

On Oahu's north shore, three Army mobile SCR-270 radar sets were in operation this Sunday morning, from 4:00 to 7:00 a.m., in accordance with the schedule established under Alert No. 1. All three (Kawailoa, Opana, and Kaaawa) recorded the approach of two Japanese reconnaissance planes, launched from cruisers, when they were about fifty miles away, beginning at 6:45 a.m. One of the stations (but not Opana) reported this flight to a Navy lieutenant on duty at the Army information center at Fort Shafter about 6:52 a.m., who reported it to another Navy lieutenant who responded that the Navy "had a reconnaissance flight out and that's what this flight was." Much better known is the report by the Opana station at 7:20 a.m. of a mass flight of planes approaching from a northerly direction. This was the first wave of Japanese bombers and fighters, which had been spotted by the Opana radar just after seven while still some 130 miles from Oahu. By the time the Opana report came in the information center had officially closed down, and an Army lieutenant who happened to be still on duty decided that nothing need be done about the call--he knew that American carriers were out and assumed that Opana had picked up a reconnaissance flight from one of them.

  • I was wondering about the radar station being operated by the army at Pearl Harbour, and the reason for that. Commented Sep 11, 2017 at 13:10
  • There was no ground based radar inside the base at Pearl Harbor. There were 5 portable radar units in Hawaii one of which discovered the Japanese planes 130 miles distant, 40 minutes before the attack. Those portable army RADAR units had just been deployed, and there use not well thought out.
    – user27618
    Commented Nov 27, 2017 at 19:32
  • Why was the Army in overall charge of the defence of an isolated chain of islands in the middle of the North Pacific?
    – Vikki
    Commented Nov 22, 2018 at 4:03
  • Also, why not send out a flight to intercept the incoming aircraft just to make sure they were, in fact, friendlies? If they're friendlies, no harm done; if they're hostiles, then the Japanese element of surprise is blown wide open and the attacking aircraft have to fight their way through every fighter on Oahu (and many of the survivors will end up having to ditch for lack of a carrier to return to).
    – Vikki
    Commented Nov 22, 2018 at 4:07
  • @Sean The army, having most of the air and ground forces, was responsible for air and coastal defense. The navy, having the naval assets, was responsible for defending the surrounding ocean. This required the two separate branches to coordinate to work out. They didn't and it didn't. As for why not send out a flight, as mentioned the US was at peace and thought an air attack on Hawaii inconceivable. This was another routine day. Unnecessary interceptor flights cost money, fuel, maintenance, and disrupt training.
    – Schwern
    Commented Nov 22, 2018 at 9:03

The failures seem to have been at all levels, but ultimately stemmed from the distribution of responsibility for planning and implementing an effective network.

In hindsight, it is easy to see that there should have been a single authority with that responsibility.

In reality, there wasn't.

In the aftermath of Pearl Harbor, Robert Watson-Watt was called in to give an assessment of what went wrong. His report was damning. He identified:

a multitude of reasons why American radar, radar sites, radar organization, radar planning, and radar operators were not good.

He found that operators were:

"ill-selected, ill-trained, inadequate in numbers, and only transitorily within [the] Western Defense Command"

and that planning at top levels was poor, declaring that:

"the distribution of responsibility for planning between the War Department and the commands was indefinite and illogical "

In his opinion, the higher commands simply:

did not realize the inherent technical limitations of radars.

THE SIGNAL CORPS: THE TEST (December 1941 to July 1943), Washington, 1957, p111

(The book is one in a series. They are available in pdf format and make for fascinating reading)

EDIT: I've carried out some further searches today, and although I found found a number of other references to Robert Watson-Watt's report in secondary and tertiary sources, I wasn't able to find an online copy of the report itself. It is possible that it is still only available as hard-copy in official archives within the US.


They weren't slow to adopt radar. My father was battalion commander of the USAAF Signal Corp unit (largely radar operations) in China during the war, and related that the US Army had begun to establish radar units as early as 1939, not too long after the British had their early radar units operational.

Given that bombs weren't falling on the US, while they were falling on Britain, and that (at least at that time) the US considered itself relatively immune from air attack due to the distance involved, the US didn't have as pressing a reason to fully implement radar in 1940. The adoption in the absence of an immediate threat was surprisingly speedy for the normally conservative US Army.

Ironically, it was Japan who was slow to adopt radar, much to their detriment later in the war.

To fully understand how Pearl Harbor happened, you must consider not just the events, but the context in which they happened, the mindset of the decision makers and military leaders in 1941.

What the US was slow to do, is believe was that anyone could mount a major air attack on US territory in 1941. That is why the radar operators on Hawaii thought the incoming aircraft were US - it wasn't possible for Japan to mount an air attack of that scale from 3000 miles away. One or two unidentified planes might have raised an alarm... Japan did send some individual long range flying boats on nuisance raids to Pearl later on, operating from islands or ship tenders. But a large formation? The US was the only country that could fly a large formation near Hawaii... or so the military leadership thought. No military had ever operated a large group of aircraft over that distance. Not until that day.

The US did think that Japan might attack some of their remote bases in the Philippines or elsewhere, which is why the carriers were gone on Dec 7 - they were delivering aircraft to Wake Island. But Pearl? Too far away.

It is also important to consider just how risky that attack was for Japan. They committed most of their carrier force to an attack on a well armed opponent, with no friendly bases or support within over 2000 miles, meaning that damaged ships would have been lost, and the fleet could be subject to counterattack for the two weeks it would take to return to Japan after the raid.

If the US had detected the fleet and ascertained their intentions, the Japanese fleet could have run into nine battleships, three carriers, and numerous smaller combat ships in mid ocean, and suffered great losses that Japan didn't have the industrial capacity to replace quickly.

This was another reason that military minds in the US didn't consider a naval air attack on Pearl Harbor seriously - it was too risky for an island nation that depended upon it's navy for all military capability, to undertake. That could have gone very badly for Japan.

The Japanese attack on Pearl Harbor was unprecedented in scope and audacity. They took a huge risk, and the fortunes of war smiled on them... briefly.

This was not a matter of not adopting radar. It was a matter of not believing what the radar told them, and not believing it for good reason - most nations would not have done something that risky.

A mindset parallel can be found in the 9/11 attacks. Prior to that, no one would have seriously thought that an attack of that cruelty, and on that scale, could be mounted by a terrorist group.


The United States was aware of the potential of RADAR for a decade before WWII began.

  • In the autumn of 1922, Albert H. Taylor and Leo C. Young at the U.S. Naval Aircraft Radio Laboratory , suggested that radio waves could be used for ship detection in a harbor defense, but their suggestion was not taken up.
  • In 1930, Lawrence A. Hyland and Albert A. Taylor at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., used radio equipment to detect a passing aircraft, five years before Watts presented this ability to the British military.
  • 1934, Americans Page, Taylor, and Young are credited with building and demonstrating the world’s first true radar.
  • Scientists for the United States coined the term RADAR an acronym for Radio Detection And Ranging".

By the mid 1930's the investment and innovations in RADAR were coming out of Britain and Germany. Why hadn’t the US military pursued RADAR harder and why wasn’t the largest economy in the world(the United States) spending the most money on this important technology?

The US army prior to WWII was not an institution which had much financial support to pursue innovative high reward projects with long research cycles like radar. The United States had the largest economy in the world for decades by the time WWII began, but the United States maintained a small poorly funded military commensurate with a historic policy of isolation and neutrality in foreign wars. In 150 years the only "foreign war"** which the United States ever entered was WWI. The policy of neutrality and isolationism which had come down from George Washington in his farewell address was one which had significant popular and political support right up until Pearl Harbor.

** By "foreign war", I don't mean a war with a foreign power. I mean a war like WWI or WWII which began as a war between two foreign powers and the US entered due to an alliance or perceived interest.

Biennial Report, By the Chief of Staff of the United States Army To the Secretary of War 1 July 1939 to June of 1945


"When General Marshal took office (Sept 1939), the 174,000-man U.S. Army ranked nineteenth in size in the world, behind Portugal and only slightly ahead of Bulgaria.

US Army Strength By Year

June of 1939...............................187,893
August of 1941.........................1,588,032
May of 1945 (peak strength)....8,291,336
document provided by James Tobias, U.S Army Center of Military History at Fort McNair in Washington, D.C. by email)

While the US economy was significantly larger than the British economy in the 1930's, the British military was approximately 5 times larger than the US military when WWII began (Sept 1939).

Why didn’t the United States adopt Radar sooner? Money and general national lack of commitment/interest in defense. The US populous and political leadership(outside the executive) was barely interested in preparing for the possibility of war.

On the eve of WWII, Congress nearly allowed 80% of the US Army to return to civilian life. The military build up being pursued by Roosevelt called for a 1 year enlistment and congress (by a single vote) just narrowly passed an extension. 203 - 202, August 1941. "the draft extension bill."

Even though Roosevelt believed war was inevitable. Roosevelt ran for office in 1940 promising to keep us out of WWII, because to do otherwise would have been politically unsupportable.

Roosevelt, acutely aware of strong isolationist and non-interventionism sentiment, promised there would be no involvement in foreign wars if he were re-elected.

The US was a committed isolationist country and the only thing that changed that was Pearl Harbor. Research on weapon systems with long development cycles just were not prioritized.

What's happening with other weapon systems?

Oh and by law the US military was not permitted to have Tank divisions. The Defense Act of 1920 restricted tanks to infantry use only; as a result, the Tank Corps was disbanded, with the remaining tanks distributed among the infantry.

So to wrap it up. The US army prior to WWII was not an institution which had much financial support to pursue innovative high reward projects with long research cycles like radar. They did not even have the financial support to pursue proven technologies like torpedoes, fighter planes, and tanks between WWI and WWII. During their massive build up of 1939-41 they were focused on overcoming the logistical chasm the US lack of commitment had dug in proven military weaponry and just providing the basics to a military which was growing 800% in three years from 1939-1941. Even this modest goal they barely had enough political support to accomplish.

  • 2
    I think you are conflating the abysmal sate of U.S. armed forces in 1917, with the merely terrible state in December 1941. In 1941 the military had already been upgrading, albeit slowly, for several years. Specific source references might point out where your description is accurate for 1941 and noe just for 1917. Commented Nov 26, 2017 at 15:17
  • When I refer to the beginning of World War II I was referring to September 1939.
    – user27618
    Commented Nov 27, 2017 at 0:37
  • 1
    Actually from 1939-1941, the United States Army had grown more than 800% in size, not slow at all. (see above for source)
    – user27618
    Commented Nov 27, 2017 at 21:30

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