The US never wanted to join World War II so they isolated themselves from everything that was going on and tried to stay neutral (Over time, we sided with the Allies). Mainly, we entered due to the Japanese attack on Pearl Harbor. After the bombing, the US decided to declare war on Japan. Soon after, Nazi Germany's leader declared war on the US and then the US was officially in the war.
So, all in all, it was the bombing of Pearl Harbor on December 7, 1941.
Hope this helps :)
Henry is described as a "war devil" in the Red Badge of Courage because he loads and shoots and reloads and shoots and shoots until his rifle is so hot. He runs towards the enemy without any regard for his life, so that’s why he is described as a madman, a “war-devil”.
