World War 1:
Was mostly based in trenches . It was the first ever war that was different to the "traditional wars" (cavalry etc.).
In rough, it started because A-Hungary made alliances with Germany, and other powerful countries. In turn, Britain, France, Russia made a rival alliance. Lots of little reasons, like the murder of Arch Duke Ferdinand caused the two teams to declaire war.
World War 2:
Basically, Hitlar kept claiming territory, when Germany was banned from doing so. Britain kept ignoring this, until Hitler's forces marched into Poland. By this time, he had become so powerful, that it took years to defeat him (And that was only because he was foolish enough to betray Russia)