When the news filtered through to British viewers late on Saturday evening that someone had shot Donald Trump, the initial reaction was shock. It is understood that American politics, even more than our own, has become bitterly polarized over the last 10 or 15 years, but overwhelmingly it has within the lines of some rudimentary norms.

On reflection, however, some thought it was not so surprising after all. There is an external perception that public discourse in the U.S. has a thread of violence, especially gun violence, running through it, something we in Britain we find difficult to imagine. The assassination of President John F. Kennedy in 1963 is, of course, the “main event,” closely followed by that of Abraham Lincoln; James Garfield and William McKinley also made the ultimate sacrifice for public service.

In a way, we are reminded of this bloody history every time we see a U.S. president, by the cocoon of security in which incumbents are wrapped. Since 1901, the U.S. Secret Service has been charged with protecting presidents and presidential candidates, and federal law prevents the president and vice president from refusing this security. Hundreds of agents provide round-the-clock security. The president is never alone, and he travels in an armored limousine known as “the Beast.”

At first glance,........

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