The damage a President Trump could do to the American republic is thankfully limited. A president is not a king. There are laws and a court to enforce them. There is Congress to block him. If a president breaks the law, he can be impeached. America may become a more intolerant and chaotic place under Donald Trump, but thanks to its institutions the republic will survive.
The world is a different matter. The president enjoys his or her greatest power in foreign policy. His power to use force is well known. As important, though, is what he can choose not to do. He can unilaterally refuse to defend an ally. He can choose to strike a bargain with Russia instead of deterring it. He can pull out of a trade deal. There are fewer checks and balances. Damage done in one year may never be undone.
A Trump administration would pose the greatest shock to international peace and stability since the 1930s. This is not because Mr Trump would invade other countries but because he would unilaterally liquidate the liberal international order that presidents have built and defended since Franklin Delano Roosevelt. If the word “isolationist” has any meaning, he qualifies as one.