My knowledge of WW2 falls somewhere between "way more than the average American" and "woefully lacking compared to the AskHistorians crowd." Can someone explain the cause/reasons behind the Japanese-German alliance? They never really coordinated as far as I can tell. My two potential reasons would be to split the Russians and Americans into a two-front war. But did Germany anticipate US entrance at the time of the alliance? And did Japan really divert much of Russia's resources?
tl;dr: they shared a major common enemy, first the Soviet Union and then the United States.
Germany and Japan started flirting with each other in the mid-30s when both countries perceived the Soviet Union as their greatest enemy, signing the Anti-Comintern Pact in 1936. However, the Japanese defeat against the Soviets at Khalkhin Gol in August 1939, combined with the rapid German victory in Western Europe in May-June 1940, enticed the Japanese to pursue their ambitions southward towards the resource-rich European colonies in French Indochina and the Dutch East Indies that they needed to furnish their war in China. Realizing this might provoke a war with the US, they decided to pacify their northern flank by signing a neutrality pact with the Soviets in April 1941 and later resisted entreaties from Hitler in July 1941 to attack Vladivostok.
Enter the US. The formal German-Italian-Japanese alliance was inked on September 27th, 1940. Earlier that same month, the US struck the destroyers-for-bases agreement with the UK, the first major sign that the US was going to throw in its lot with Britain. This was especially worrying for Germany since they had already unofficially penciled in an invasion of the Soviet Union for spring 1941 and were worried (correctly, as future events would confirm) about fighting the Soviet behemoth to the East while the American juggernaut attacked from the west. Japan meanwhile invaded the northern half of French Indochina on September 23rd, the first act of a Japanese "southern strategy" that was bound to antagonize Washington, as already described above.
Germany and Japan thus saw a looming threat on the horizon and decided that if war with the US were to come, it would be better to tug at it from both directions than let the US, with its industrial might, destroy each country separately.
Sources:
Ian Kershaw's Fateful Decisions: Ten Decisions That Changed the World, 1940-1941
Antony Beevor's The Second World War
More can be written, but you might like to start with the FAQ section "Axis alliance and post-war plans" , specifically
This is not to discourage discussion. More questions, data, and debate are welcome.