You could attempt to adopt a strategy of always following your commitments. From your current perspective this is useful but once you have learned your strategy has failed, what's to prevent you from just disregarding the strategy?

Disregarding it once will convince yourself and others that you will disregard it in the future, and remove your ability to make other precommitments.

The nuclear war example is more complicated, because presumably having a nuclear war will be the last thing you ever do. I would credit it to evolved instincts. Evolution "knows" that precommitments are important, so it gives us the desire to follow them even when it is not immediately rational to do so - for example, a lust for revenge that ought to be sufficient to make us retaliate in nuclear war, or a concept of "honor" that does the same.