How did Americans get into World War 1 and how did Americans get into World War 2?

Answer 1

What ultimately garner American public support for the First World War was the Zimmerman Telegraph, in which Germany supposedly asked Mexico to declare war on the US, and, in exchange, offered them the treaties which Mexico had ceded to the US with the Treaty of Guadalupe Hidalgo.

The default answer regarding the Second World War is the attack on Pearl Harbour. In reality, the US government had been looking for a reason to enter into the war, and put pressure on the Axis forces to force their hand to do something which would garner the support of the American public for entry into the war. The Lend-Lease Programme and the oil embargo against Japan are the two primary means the government sought to achieve this.

To remark on your first response to when America entered world war one, I would have guessed the Lusitania incident next to the Zimmerman Telegram intercepted by the British, but porportedly falsified to force the US into the war giving the Allies a surge of support.