[quahada]

In addition to the other points made on this thread, I think a big factor is that winners don't gain much from war, so why bother.

Historically, when economies grew at the rate of population, the only way to become wealthier is to conquer more land. Now, with the global/knowledge based economy, you can become wealthier without having to take over someone else's land. In fact, the costs of the war probably outweigh the spoils.