What was it that triggered countries to fight in world war?

Answer 1

Japan attacked the US because they wanted to disable the US Naval Fleet so they could not prevent Japan from invading the Dutch East Indies for their oil. The US had placed an oil embargo on Japan for invading other countries for their oil and other resources.

In the European theater at the end, of the Great War (WW 1) the French and many other countries blamed Germany for the war. Rather, than accept the idea that the war was really started by Austria/Hungary (and their appeal to Germany to help them in the war) they laid the entire fault on Germany. They made the Germans give up all military power and pay millions to the other countries which were so badly damaged. There were 14 points to that treaty. The Treaty put Germany into a serious depression and economic downfall. The Kaiser left his ruling position and many decades were filled with conflicting political parties. This set the stage for World War 2.

Adolf Hitler became Chancellor of Germany. He was like a vice president. When the leader of Germany died Adolf Hitler took over legally. He then abolished all adherence to the Versailles Treaty and set up NAZI rule (The Third Reich). He intended on annihilating all the Jews since he blamed them for all Germany's troubles and he hated them. So he came up with the Final Solution to get rid of all people who were not pure Aryan people and were enemies of the government (him really). He chose to start with the Polish People. He invaded Poland. He removed the Polish People and moved many German People into Poland. This incensed the French and the British (and other countries). That was the reason World War 2 started.