Japan attacked the US, and the US declared war on Japan. Japan had a treaty with Germany -- as reported in "The Guardian":

Thursday November 26, 1936

The German-Japanese Treaty was signed this morning by Herr von Ribbentrop as extraordinary plenipotentiary of the Reich on behalf of Herr Hitler, the Chancellor, and by Viscount Mushakoji, the Japanese Ambassador in Berlin.

As I reported last week, it is directed against the Moscow Communist International. The treaty is a document containing three articles and a supplementary protocol of three paragraphs. It provides for an exchange of information between the two Governments upon the activities of the Communist International and for consultation and co-operation in measures of defence against it. Provision is made for the adhesion of other States to the agreement. A permanent commission is to be created for common action against the International. The two States are to take within their own laws "severe measures against those who, at home or abroad, are working directly or indirectly in the service" of the Comintern. The treaty's term is five years.

The most interesting parts of the treaty are the second article and the third paragraph in the protocol, providing respectively for the adhesion of other States and for the permanent commission, which, it is believed here, makes the treaty much more than a simple police agreement. Here is seen the technical means for the expansion of the "anti-Bolshevik front."

Apart from the dependencies of the three anti-Bolshevik Powers, Germany hopes for and aims at obtaining the adhesion of Rumania and possibly of Poland. To detach Rumania from her agreement with Russia, and completely to undo the work of M. Titulescu, would, from the German view, be in itself a great triumph, and, additionally, a big step towards the bigger and remote goal of getting rid of the Franco-Russian Pact, if and when a favourable opportunity should occur, through the formation of a Government of the Right in France.

The importance of the standing commission is that it is capable of keeping the German-Japanese treaty very much alive by what might be described as a species of permanent political general staff discussions.

In 1931, Japan had invaded Northern China. The rationale, as reported in "Time" (August 9, 1937):

...in a speech to the Diet last week by Premier Prince Konoye. "I think there are many persons in the Chinese Government who understand Japan, including General Chiang Kai-shek," purred the Premier. "I think it should be the basic keynote of Japan's China policy to make the Chinese race and the Chinese Government return to their original nature as an Oriental people." After explaining that Communism is un-Oriental. while tactfully omitting to mention that the Chinese Communists have now tentatively joined forces with the Chinese Government. Japan's Premier blandly added: "For China to dance to such a [Communist] tune and bring on trouble in the Orient is tantamount to weakening the Orient by its own hands. I earnestly hope that the Chinese race will awaken as quickly as possible to realization of its nature as an Oriental race and that it will cooperate with the Japanese, who come of the same Oriental stock. . . . Japan wants not territory but cooperation. ... If we had such designs, the entire territory of North China would have been seized by the invincible Imperial Army long ago."

In this Japanese pronouncement connoisseurs observed the authentic Adolf Hitler touch, chalked it up to the German-Japanese Treaty signed last year for co-operation against Communism.

Thus, the invasion of China by Japan was not to seize China, but to defeat Mao and his Communists. Imagine how surprised the Japanese must have been when the Chaing Kai-Shek's army fought back! Note too that China did not declare war on Japan until July 7, 1937.

Two years after this, Germany invaded Poland. Poland had mutual defense treaties with France and Britain, so Britain and France declared war on Germany. Germany, meanwhile, had cut a deal with Stalin (on August 23, 1939), the German-Soviet Nonagression Pact of 1939, which in effect gave the Soviet Union a chunk of Poland for not attacking the attacking German forces.

Since Germany and Japan had the aforementioned treaty, Japan and Italy (remember Italy?) also went to war with Britain and France.

Of course, a year later Hitler tossed this treaty out and attacked Russian on June 22, 1941, and Stalin was surprised even though he'd been told, from both British and Japanese sources, that the attack was coming.

In December, the Japanese attacked the US at Pearl Harbor and the US declared war on Japan. Again, because of the 1936 treaty, the Germans and Italians declared war on the US.

Here's a brief timeline:

December 7: Japan declared war on the United States, United Kingdom, Australia, Canada, New Zealand and the Union of South Africa. United Kingdom, Australia and New Zealand declare war on Finland, Hungary and Romania. Canada declared war on Finland, Hungary, Japan and Romania. Panama declared war on Japan. Yugoslavia at war with Japan.

December 8: United States, United Kingdom, Australia, Costa Rica, The Dominican Republic, El Salvador, Haiti, Honduras, The Netherlands, New Zealand and Nicaragua declared war on Japan.

December 11: Germany and Italy declared war on the United States. The United States and China declared war on Germany and Italy.

Avoid entangling foriegn alliances, George Washington advised in his Farewell Address.

He was right.