WWI mainly developed in Europe. It only included the USA because Germans attacked a US ship. What did Japan have to do with Europe?
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.
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:
- Japan sought expand its power by limiting Russian influence in North China, and hoped to acquire control of German colonies in China.
- 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.
Why did WWI include Japan?
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.
1879 - Empire of Japan annexes the entire Okinawa Archipelago, (Ryukyu)
July 1894 – April 1895 The First Sino-Japanese War, left Japan with overseas possessions in Korea, China.
May of 1896 - The Japanese invasion of Taiwan allowed Japan to add Taiwan to it's territory taken at China's expense.
1904-1905 The Russo Japanese War of , Japan eliminated Russia as a potential rival checking it's expansion in the far east.
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
- Part of New Guinea
- Micronesia - Given to Japan after WWI
- Solomon Islands
- Bougainville Island
- Marshal Islands - Given to Japan after WWI
- Northern Mariana Islands - Given to Japan after WWI
- Saipan - Given to Japan after WWI
- Caroline Islands including (Palau) - Given to Japan after WWI
- Samoa - Given to Japan after WWI
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.