If you think that the president of the United States is the most powerful individual in the world, then worry about Donald Trump is justified. If you think that America is still the world's number one superpower — and that we still live in an age of superpowers — then carry on worrying. If you think that America is the world's only policeman and that without deep American involvement the Middle East will descend further into chaos then worry some more.
But just suppose the world is not like that anymore. Suppose that the U.S., while still a giant economy full of dynamism and innovative vigor, has to share its power with a whole network of other international "actors," groups and interests, and that merely spending seven times more on defense than its closest allies, seems not to be able to get its way or clinch settlements round the world.
Suppose that the U.S., in a totally connected global system, is just as constrained in its independent actions as almost every other country. And suppose that, anyway, the office of the president is designed, and has been from the birth of the U.S., to constrain presidential power in a maze of checks and balances, which are now vastly reinforced by the web.