Not a demo, but the full game, with 5 free hours of play. Strategic Command: European Theatre is a hex-based computer wargame in the classic mold. Covering the entirety of the war with scenarios that allow you to start in each year from 1939 on, at a scale of 50 miles per hex, Strategic Command gives a solid feeling for the ebb and flow of the conflict. One of Strategic Command's best features, which improves its repeat playability, is the ability to turn on randomness for political events.
Doing so means that countries other than Germany, France, and the UK declare war on a schedule that generally diverges from the actual events--so that, with luck, Germany might not face a war with Russia until later, or that Italy might not enter the war at all. This is a relatively simple wargame, one that should be accessible to people who are not hardcore wargamers, though reading the manual is advised.