Hurting friends and helping enemies

Donald Trump's decision to declare victory in Syria and pull US troops out of the country is shocking on many levels. Perhaps the greatest shock is that it undermines one of the few, hitherto unshakeable, principles of how the world works: there are no friendships in diplomacy, only national interests. In suddenly leaving Syria, Trump, above all, threatens the United States' core interests: he is throwing away battlefield gains, leaving his country without any leverage against global rival Russia and regional rival Iran; he is subverting every alliance that America has and will have; he is trashing the authority, expertise, confidence and sacrifices of America's soldiers, diplomats and intelligence services; he is making the world more dangerous, to the detriment of all, including the United States.

The principle that all powers care only for their own interests is the...