Imperialism. militarism, and the alliance system.
chocolate
Germany declared war on Poland. Poland didn't have a choice.
isolationism and neutrality
Maybe you haven't heard about the Japanese attack on Pearl Harbour on 7 December 1941.
so they can help out britain and not get taken over
One reason is the Assassination of Archduke Ferdinand
It helped win the war
The USA joined World War I after the Germans sank the Lusitania, a British cruise-liner, containing hundreds of Americans. Their entrance is similar to Belgium's, as they were attacked by Germany. Other countries entered the war for reasons of alliances or reasons of wanting to gain land.
It was one of the 4 major reasons of war, the other three are Alliances, Militarism, Nationalism.
No war without cause.
it would kill many people
Economy great depression and something else