Military agression of Japan, Germany, and Italy?

Short answer: The US tried to stay neutral, but its clear support for Britain pushed it to support the Allies even before Pearl Harbor.

In Germany during the 1920's, massive inflation, unemployment, and resentment pushed radical political parties on both sides of the spectrum forward, and by 1933, the Nazis had gained control. They quickly removed all opposition parties and cracked down on gays, Jews, political dissidents (mainly communists, Marxists, and socialists), and other "undesireables." First, Hitler annexed Austria, then moved on Czechoslovakia. France and Britain allowed this at the Munich Conference under the agreement that Hitler would make no more territorial demands. This, however, did not come to pass, and on September 1, 1939, World War Two began with the German invasion of Poland. Britain and France declared war, but on May 10, 1940, German panzers thundered through the Ardennes Forest, splitting the Allied armies and ruining any chance of saving France. With Britain the only ally left to the US in Europe, Roosevelt's Congress passed Lend-Lease, which allowed for huge material support of England.

At this point, the US began shipping massive amounts of military supplies (trucks, tanks, antiaircraft guns, weapons, ammunition, bombs, etc) to the British to hold off the German onslaught. This paid off when the British succeeded in halting the planned German invasion of England, codenamed Operation Sea Lion. Lend Lease continued to heavily assist the British until the US fully entered the war after Germany declared war on the US following Pearl Harbor.

We should now turn some attention to Italy, even though it was not nearly as significant as Germany. Italy had first pushed at the British in 1936 when Mussolini began to try to take the area from Libya to Ethiopia. From 1936 to 1940, the Italians were unable to take this area alone, so Hitler and the Germans stepped in with the Afrika Korps (led by the formidable Erwin Rommel) to end the British presence in Egypt (in between Libya and Ethiopia). Although the US responded less directly to this threat than to the Germans, Lend-Lease again played a huge role in helping the British, and American M-4 Sherman saw combat under the British in North Africa before the US was even in the war.

Japan presented a different challenge, however. Because the US lacked allies capable of resisting the Japanese in the area, all that the US could do was fight economically. When Japan began making land grabs, the US cut off oil sales to Japan, pushing the Japanese to move further south to get oil from Dutch-held areas. As Japanese expansion continued, the US Navy began a massive rearmament project (along with all the other branches) to prepare for war should it come, and on December 7, 1941, the Japanese Imperial Navy attacked Pearl Harbor, destroying most of the US battleship fleet and severely hindering the US Navy's fighting capability. Fortunately for the US, aircraft carriers were out of port on Dec 7; thus, they were spared and by default became the spearhead of US naval efforts in the Pacific.

I love history, so I'm sorry if I wrote too much and helped little. I'm guessing that you can pull SOMETHING out of what I wrote, however.

The U.S. originally tried avoiding the aggression of the Axis powers, however were forced into the war by the attacks on Pearl Harbor.