Answer:
Imperialism
Explanation:
I am not personally sure if this 100% correct but from my understanding most countries go to war as a result of a desire to expand their influence and control over other nations for resources or wealth and overall political influence, which can be demonstrated by the many empires throughout history that conquered their neighbors to gain access to more resources, (sometimes people), and more control. This can also be seen notably by the British's efforts of creating colonies overseas which usually results in conflicts by the native population.
Three reasons WW2 is significant:
1) It was basically a continuation of WW1 (which was supposed to be the war to end all wars)
2) It basically got final revenge on Germany for the allied side
3) America was basically pulled out of their word to not get involved into the world affairs.
2)
"a proposal by a german official that mexico attack the US"