World War 2Why did Singapore enter the World War 2?
During WWII, Singapore was not an independent country. It was part of the British Empire. Because of that, Singapore itself did not enter WWII. It was automatically made part of the war because Britain was involved.
If you are asking where there was a a battle for Singapore, Britain had tried to make Singapore their major naval stronghold in the area. This ensured that Japan would attack Singapore, which is why Singapore became the site of a battle.