WWI mainly developed in Europe. It only included the USA because Germans attacked a US ship. What did Japan have to do with Europe?

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    You wrote "Germans attacked a US ship". What ship was that? Commented May 4, 2019 at 13:28
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    @kimchilover probably meant the Lusitania, which was not a US ship, but which had many US citizens on board
    – Stephen S
    Commented May 4, 2019 at 18:30
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    There was more than one provocation toward the US by Germany before the US got involved, the Zimmerman telegram, for one.
    – Davidw
    Commented May 5, 2019 at 1:56
  • @Davidw Not to mention about ties between England and the U.S.
    – MilkyWay90
    Commented May 5, 2019 at 2:14
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    "Discussion" however, is not something to be carried out in comments. Their purpose is entirely for clarifying or improving the post. Off-topic Discussion has been deleted.
    – T.E.D.
    Commented May 9, 2019 at 15:24

4 Answers 4


Japan had been an ally of the United Kingdom since the Anglo-Japanese Alliance of 1902. That alliance had been renewed in 1905, following the Treaty of Portsmouth that formally ended the Russo-Japanese War of 1904-05, and again in 1911.

The impact of the alliance was that it facilitated Japan's entry into the war on the side of the UK and her allies (although the treaty did not actually require Japan to do so). This is discussed in Chapter 3 of The Anglo-Japanese Alliance by Alfred L.P. Dennis.

The sequence of events is clearly explained in the Wikipedia article Japan during World War I:

In the first week of World War I Japan proposed to the United Kingdom, its ally since 1902, that Japan would enter the war if it could take Germany's Pacific territories.4 On 7 August 1914, the British government officially asked Japan for assistance in destroying the raiders from the Imperial German Navy in and around Chinese waters. Japan sent Germany an ultimatum on 23 August 1914, which went unanswered; Japan then formally declared war on Germany on 23 August 1914 in the name of the Emperor Taishō.

As the article goes on to explain:

As Vienna refused to withdraw the Austro-Hungarian cruiser SMS Kaiserin Elisabeth from Qingdao, Japan declared war on Austria-Hungary, too, on 25 August 1914.

Essentially, Japan hoped to extend its sphere of influence in the Pacific. In a sense, this can be thought of as a continuation of Imperial Japanese policy in the region that had led to the Russo-Japanese War a decade earlier.

  • 8
    Note also that ceding Germany's lands around Qingdao directly to the Japanese instead of to the Chinese (1) was the beginning of the proper Japanese beer industry and (2) pissed off the Chinese - who also joined the war on the Allies' side - so much that the May 4 movement remade its culture, including the launch of its Communist Party and revision of its literature. It's the hundredth anniversary this year, and the lesson that might prevails and international organizations can never be counted on to do the right thing is still very deeply felt in the country.
    – lly
    Commented May 4, 2019 at 8:29
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    What "allies"? Germany, Turkey and Austria-Hungary were also allies (of each other). You might mean Entente? There was no side called "Allies" in WW1.
    – vsz
    Commented May 5, 2019 at 12:25
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    @vsz actually, I meant "the UK and her allies", since the treaty was just between Japan and the UK. I've edited to clarify. However, although the Triple Entente (the UK, France and Russia) was the core of the Allied Powers in WW1, they were not the only members. In fact, the shorthand term 'allies' is commonly used to refer to the Allied Powers in both world wars. Commented May 5, 2019 at 12:33
  • @LangLangC Let us continue this discussion in chat. Commented May 5, 2019 at 18:27

The answer by sempaiscuba is very good. I just want to discuss the background of the situation a bit.

Around the 1860's, Japan had to grant extraterritorial rights to various European powers that were engaged in colonialism. Many people in power in Japan were well aware of what the European powers had done to China before, during, and after the Opium wars. Not only did the Japanese aristocracy not want to see their power eroded by European colonial regimes, they were well organized enough to set about a program to eject the Europeans from Japan. They did this by first studying the structure of Euro-American economies, then restructuring Japanese society along western lines from the ground up.

By about 1900 Japan had modernized to where it was able to end extraterritorial rights and even had a colony of its own (Taiwan). However, the Japanese leadership was well aware of two things. Firstly, they were aware that it could still be economically then politically subjugated to the European powers. Secondly, that colonialism had the potential to greatly expand the wealth and power of the Japanese aristocracy. Japan would come to dominate Korea as a colony, and worked to prevent its European neighbor, the Russian Empire, from controlling a warm water port in China during the Russo-Japanese war.

Of its many objectives during WWI several are easy to identify:

  1. Japan sought expand its power by limiting Russian influence in North China, and hoped to acquire control of German colonies in China.
  2. Japan wanted to build relationships with European allies to expand its own power.

Considering that there was almost no downside - Germany was in no position to invade Japan - it made sense to join the war effort.

  • +1 for last sentence!
    – ed.hank
    Commented Apr 3, 2020 at 11:44

Why did WWI include Japan?

Short Answer

In the late 19th and early 20th centuries or the Meiji Period(October 23, 1868 - July 30, 1912), the Empire of Japan underwent a rapid expansion; fueled by a feudalistic military based economy and European technology (largely from Britain). Japan chose to enter WWI because it saw an opportunity to continue grabbing south pacific possessions now from a European Country with minimal risk to itself. Japan successfully accomplished this goal adding considerable to it's territory as a member of the victors of WWI.

If you read the lists of the German possessions they read like a road map of WWII battles.


Detailed Answer

1914 and WWI was just just more of the same. Japan saw a chance for a land grab and getting a share of Germany's sizable Pacific possessions with minimal risk to itself given Germany's preoccupation elsewhere. That's what Japan did.

Before WWI Germany owned

Japan during WWI concentrated on invading and occupying these German possessions, and then at the end of WWI was given a share of the possessions as a member of the victors of WWI.

Japan's WWI booty was dubbed The South Pacific Mandate


Japan was a British ally and wanted the colonies that Germany had in the Pacific, read Japan during WWI.

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