[yfkar]

You're assuming a narcissist behaves rationally and thinks long term. Trump's overinflated ego causes him to vastly overestimate his abilities (eg. "I know ISIS more than the generals do") and makes him unable to admit blame or failure (eg. if I lose the election, it'll be because it was rigged), which is a terrible combination. If he was rational and wanted his legacy to be good he would improve on constructive criticism, but based on his campaign he seems unable of taking any kind of criticism. Trump is already remembered as the candidate for president who had a habit of starting unnecessary twitter feuds whenever he felt insulted. If Trump could do whatever he wanted as president, he would ban everyone from criticizing him. If he has to choose between a long-term benefit for the country and a short-term personal benefit, he will choose the personal benefit.

As for not causing unnecessary military conflicts: "And, by the way, with Iran, when they circle our beautiful destroyers with their little boats, and they make gestures at our people, that they shouldn't be allowed to make, they will be shot out of the water."