Why did we join world war 2? and what was the effect of it?

World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.              

The U.S. joined WWll i believe in 1945. We joined due to the attack on Pearl Harbor. Because the Japenese bombed Pearl Harbor and were allies to Germany, we joined the war because they were a threat to our nation. Also, we joined the war due to Hitlers ideas of world domination. Hitler had started to gain power and was not stopping at any cost. So this also posed as a threat to our nation.