I suppose WW2 started when Germany invaded Poland. Britian had treated to declare war on Germany if it did so, and thats just want happened. You could agrue that WW2 started before that, i.e when Germany invaded other Germanic speaking areas in Europe but most countries thought fair enough, they are German speaking, and their was some hard land reductions on Germany after WW1.
Check out the link
WW2 letters from soldiers for more information about WW2.