The Forever War

The best and the brightest gathered and trained to fight off a mysterious alien threat. In some ways The Forever War sounds kind of like Ender's Game, except rather than spending an entire book training these recruits get sent out into the fray to fight and deal with the repercussions of time dilation as they travel to more and more distant planets. Weeks may pass for the soldiers, but by the time they arrive at their next battle their enemy may have been preparing for hundreds of years with newer technology and tactics.

Perhaps not as thought provoking and Ender's Game, however The Forever War offers a fascinating perspective on the implications of fighting a battle across light years where the next skirmish could be with an enemy that, in some senses, is from the future.