It didn't take long for London's new mayor to make a splash on the world stage: He knocked a fast but effective hole in Donald Trump's proposed temporary ban on foreign Muslims entering the United States.

In many ways, of course, it wasn't a particularly meaningful victory. Trump may never reach the White House. Even if he does, how he might implement his ban is still far from clear. Trump himself now says Sadiq Khan, the first Muslim to become mayor of a major Western city, would be an "exception" to his proposed policy. But it is not clear what that means, either.

Khan, for his part, says he would not visit the United States, on principle, if the broader ban were in effect.

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