Why did the war widen to become a world conflict?

Which War? :)
WW1, WW2, The Cold War, War on Terrorism... etc etc. [sorry but details would offer a better answer]

- If inre a World War the second Britain got involved the wars became global (the Commonwealth got involved also, Australia, Canada, trade and naval warfare in the Atlantic and Pacific), in WW1 that was in 1914 and in WW2 it was in 1939
- If this is an American school they'd probably ignore that fact of history and say it became a world war when they got involved.. in WW1 that was in 1917 and in WW2 that was in 1941