Several western Allied governments placed embargoes on Japan in response to its actions in China. I assume, though I'm not certain, that it contributed to Japan's decision to declare war against the United States and the British Empire. I can understand why allied governments had a dim view of Japan's actions, but I don't understand why they decided to get involved.

Wasn't their (except technically for the USA) war with Germany and Italy such a monumental undertaking that being at war with Japan at the same time too risky a move to make? Why didn't they decide to ignore the Japanese government for a while, until Germany and Italy were largely or totally defeated?

  • Some countries did as you say, notably the USSR. – user3344003 Dec 7 '15 at 16:22
  • @user3344003 by attacking the Republic of China from North-West? Xinjiang War (1937) – bytebuster Dec 7 '15 at 23:40
  • @bytebuster By keeping own interest, of course. SU tried to secure its eastern borders, while USA tried to defend own Pacific colonies. SU didn't attack RoC, but rather supported Xinjiang local government - China was such a mess at that times. – Matt Dec 8 '15 at 7:55
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    This should probably be asked on History.SE. Politics is more for questions about current systems. – PointlessSpike Dec 9 '15 at 9:43
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    I think it's more the history of current political systems that are preferred here. You're not asking for an explanation of a current system using history, you're asking about events in history. To put it another way, you're primarily advancing you're understanding of history, not current politics. – PointlessSpike Dec 9 '15 at 10:02

Because by 1940, the Japanese invasion of the Dutch East Indies became imminent.
These colonies were rich in oil, so such an invasion would ruin the energy stability of Europe and the U.S.

The Japanese invasion of China started in 1937 and cost about 30 million lives; it was associated with use of chemical weapons, forced labor camps, medical experiments on prisoners, and many other acts. However, these actions did not get a response:

On 12 September 1937, the Chinese representative, Wellington Koo, appealed to the League of Nations for international intervention. Western countries were sympathetic to the Chinese in their struggle… However, the League was unable to provide any practical measures; on 4 October, it turned the case over to the Nine Power Treaty Conference.Wikipedia

For example, in U.S. the policy of Non-interventionism was very popular. In a 1940 speech, Roosevelt argued,

"Some, indeed, still hold to the now somewhat obvious delusion that we … can safely permit the United States to become a lone island … in a world dominated by the philosophy of force."

It is important to know that Japan not only invaded China; they also invaded and occupied Indo-China, Malaya, and Philippines, gradually moving South. Japanese troop build ups in Hainan, Taiwan, and Haiphong were noted.

Japanese strategists also had confirmed plans to take control of Dutch East Indies. And they actually did it January 1942.

In September 1940 Japan was assured that Germany and Italy would respect Japanese interests in the Asia-Pacific region by signing the Tripartite Pact.

On 24 July 1941 Roosevelt requested that Japan withdraw all its forces from Indochina. Two days later the U.S. and the U.K. stopped selling oil, iron ore, and steel to Japan.

As we know, instead of withdrawal, the Japanese government began planning military actions against the Allies.

Summary. Opposing Japan was not very popular in the U.S. and Europe, but the emerging threat of losing control over oil/iron resources plus a chain of escalating events embroiled the Western Allies in opposing Japan.

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    The Japanese invasion of the Philippines started only after Pearl Harbour: en.wikipedia.org/wiki/Japanese_occupation_of_the_Philippines The threat on that American colony was part of the US concern about Japanese expansionnism, but the invasion itself should not be listed among the casues of the blocade by US and UK. – Evargalo Oct 25 '17 at 7:19
  • Malaya & Philippines at least were post Pearl Harbour. – Russell McMahon Mar 18 '20 at 10:25

To add to @bytebuster's excellent answer, Japan was threatening (with addition of Greater East Asia Co-Prosperity Sphere) to turn into a geopolitical powerhouse, spanning a large geographic area and commanding lots of natural resources AND human ones. It's never in the interest of established geopolitical powers (in this case, Western allies) to allow such to come into existence - same reason USA opposed Russian/Soviet domination of Western Europe, or earlier, Germany's domination of Eurasia.

Once you are a large geopolitical power, you possess enough resources to make it very hard to knock you off your perch - which means the best, or even only, strategy, is to check an emergent power before it expands sufficiently.

  • How much of an industrial base did Japan have compared to established world powers? – Andrew Grimm Dec 7 '15 at 20:59
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    @AndrewGrimm: Enough to produce a navy that could soon have become equal to the pacific flotilla of the US within years. – Philip Klöcking Dec 8 '15 at 13:26
  • Japan NEVER had the resources to beat the US long term. eg Yamamoto was aware of this. The best they could have hoped for was to have carved out enough territory and been enough of a nuisance that a negotiated settlement would have been acceptabvly less expensive to the US than what instead happened. Similar to Hitler's position with UK where a negotiated peace was a very real prospect until the old warmonger Churchill stepped in to set in motion events that eventually saved the day. Thank God for Churchill! :-) – Russell McMahon Mar 18 '20 at 10:28

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