Why is it said world war 2 is different than any other war?

It was a global war.

ANSWER

As a veteran of the Korean War I've lived through 5 wars starting with World War 2 and for me there are three distinct differences with World War 2 and the others. First we were fighting against two separate countries, Germany and Japan. Secondly we were attacked by Japan at Pearl Harbor and drawn into that war. Thirdly, Germany declared War on the U.S. so we didn't have much choice there either. There might be other differences but I see these three as major differences.