The main event that triggered America's entry into World War II was the surprise attack on Pearl Harbor by the Japanese on December 7, 1941. This attack led to significant losses for the U.S. Pacific Fleet and galvanized public support for entering the war. The following day, December 8, 1941, President Franklin D. Roosevelt delivered his famous "Day of Infamy" speech, and Congress declared war on Japan, marking the official entry of the United States into the conflict.
German submarine warfare
Election of Lincoln
The Japanese attack on Pearl Harbour 7 December 1941.
Confederates fired on Ft. Sumter.
Triangular trade
German submarine warfare
The bombing of Pearl Harbor by the Japanese.
The Japanese bombed the US naval base at Pearl Harbor, Hawaii on 07 Dec 1941
Invasion of poland
invasion of poland
Hitler's invasion of Poland in 1939.
Germany's invasion of Poland.
The event that triggered World War 1 was the assassination of Archduke Franz Ferdinand of Austro-Hungary on June 18, 1914 in Sarajevo.
German invasion of Poland 01 Sept 1939
the atomic bomb
pearl harbour
You have not told us which event, or which revolution you are asking about.