In December of 1941, Japan launched an attack on America at Pearl Harbor, worried that if they didn't do something quickly to cripple the American fleet, they would be starved of oil slowly and lose if America eventually entered anyway.
Oddly, Germany then declared war on America to support its Axis ally.
Both countries underestimated American manufacturing capacity.
In April, the Doolittle Raid bombed Tokyo, in May the world saw its first combined air-sea battle, the Battle of Coral Sea, and in June the U.S. Navy intercepted a Japanese invasion fleet heading for Midway Island.