Answer :
#1
Main factors are
- The colonisation contest between western countries.
- The main reason behind this was the greedyness of Britain and France with respect to Germany.
- They started war in Balkan region and led that to world war .
#2
The attack of Germany submarines on the American destroyers force America to enter world war 1.
#3
- The war raised the social dissimilarities much more than before.
- Women protested for same rights after war
- Civil war started .
#4
- Treaty of Versailles was only made to favour England and France .There was no share for US.
- So US objected that.