Which world war ii event occurred first?
the united states entered world war ii on the side of the allies.
the japanese bombed pearl harbor.

The Japanese bombed Pearl Harbor Before this event, the American government adopted a policy of isolation as it struggled with the cost of World War I and the economic problems brought about by the Great Depression.  At first they didn’t want to get involved in the conflict in Europe but they realized that eventually America would be drawn into the war.  With the bombing of Pearl Harbor, Americans were attacked by the Japanese in the hopes of destroying their ships and aircraft carriers.  They failed to destroy Americas industries that were capable of mass-producing weapons for the war.  As a result the Americans recovered and joined the war on the side of the allies.  This changed the tide of the war which led to a victory for the allies.