There was a lot of reasons.
Hitler
Cold War
Depression
It was a bunch of stuff.
Hitler starting to attack countries.
If you are talking about the U.S involvement it was the Japanese bombing of pearl harbor
the bombing of Pearl harbour on Dec.7th 1941
The violent event in 1914 helped start World War 1 was the assassination of Franz Ferdinand.
Most historians agree that the German invasion of Poland on September 1, 1939, was the event that marked the start of World War 2.
The first world war started because a group called the Black Hand Society {who were serbs} assassinated the Archduke franz Ferdinand {who was in line for the throne of Austria-Hungary} and his wife, Sophie, Duchess of Hohenberg. The person who killed him was called Gavrilo Princip.
Militancy triggered by the cultural and territorial encroachment resulting in the Creek War.
German invasion of Poland 01 Sept 1939
invasion of poland
Invasion of poland
Hitler's invasion of Poland in 1939.
Germany's invasion of Poland.
German submarine warfare
The event that triggered World War 1 was the assassination of Archduke Franz Ferdinand of Austro-Hungary on June 18, 1914 in Sarajevo.
The breaking point was when Germany invaded Poland on September 3rd 1939
The Assassination of Archduke Franz Ferdinand of Austria triggered the start of the Great War (World War I)
the assassination of the archduke Franz Ferdinand and his wife. they were assassinated by a Serbian separatist
The shooting of the Archduke of Austro-Hungarian Franz Ferdinand.
The assassination of Archduke Franz Ferdinand of Austria-Hungary by Serbian nationalists.