Germany attacked its largest trading partner, France. More than once. People do not always act rationally or in their best interests. This is why deterrence is important. Rational people don't want to fight wars.
The recent forever wars are driven by neocon thinking. Neocons are statists (people who believe the state has the best interests of the people and are the best solvers of problems) who extend domestic agenda to international agenda. Hence, Iraq limited war (Bush I) gets turned into a project to rebuild Iraq into a western liberal democracy (Bush II). Afghanistan grows from a successful special operation to a futile state rebuilding exercise.
When you're the only super power, the way adversaries beat you is by getting you to punch yourself in the face by over reacting, acting out of pride, or giving in to fear.