Along with many Americans, most Britons have looked on with astonishment and bewilderment as Donald Trump has risen to become the presumptive presidential nominee for the Republican Party. From across the pond, the success of a man so obviously unqualified for any form of public office is difficult to comprehend.

In a major break with convention, leading British politicians have openly criticized the presumptive Republican nominee. In the wake of Trump's proposed Muslim travel ban, Prime Minister David Cameron described the businessman's remarks as "divisive, stupid and wrong." Trump hit back by threatening the special relationship between the U.S. and the U.K., a crucial component of global security for most of the 20th and 21st centuries.

The new mayor of London Sadiq Khan has engaged in a war of words with Trump, describing his views as "ignorant, divisive and dangerous." Many Londoners felt proud for having elected a Muslim son of a bus driver to lead their city: It had the added bonus of feeling like a rebuke to Trump and his politics of fear. Khan has invited Trump to London to meet his Muslim family; Trump described Khan's words as "ignorant" and "nasty."

In truth, most British politicians simply do not believe that Trump will become the next president. They are comfortable criticizing him precisely because they believe there will be no diplomatic consequences for doing so. At the same time, there is some domestic advantage to be gained from being seen as standing up to him.

Yet the reality is that Trump's name will be on the ballot in November. Hillary Clinton has high unfavorable ratings with many voters. And Trump does speak for a part of American society that has been left behind. His fierce rhetoric resonates with people in distress, facing falling or stagnating wages, rising mortality and the elimination of hope for a better future. It's unlikely but not impossible that this carries Trump all the way to Pennsylvania Avenue. So what does all this mean for Anglo-American relations?

As America's closest ally and friend, we are puzzled by Trump's decision to praise our common opponent, Russian President Vladimir Putin. Putin has backed the Bashar Assad regime in Syria; invaded and occupied Ukraine in the greatest act of aggression on the European continent since World War II; and resumed strategic bomber patrols and fighter jet incursions into NATO territory. It takes the logic of "keep your enemies closer" to perversity. We think this is bad for the security of Americans, just as it threatens our own security.

Trump has called for Europeans to pay more for their own security at precisely the time that the spending cuts demanded by austerity programs make such a step-up in defense spending impossible for many countries. (Britain is the exception in committing to the NATO target of 2 percent of GDP spent on the military.) He has threatened the NATO alliance, which is the linchpin of global security. Naturally, most Britons and our European partners think a Trump presidency would be a disaster for the Western world and a great victory for our common enemies.

If Trump is elected, most Britons will want to take a timeout on the special relationship. British and other allied leaders will be in no hurry to get to Washington, D.C. to meet with President Trump. In most countries around the world, the Trump brand stands for intolerance, bigotry and stupidity. It is politically toxic. The traditional jostling to be the first foreign leader to be invited to the White House won't happen. Come January, President Trump might find many foreign leaders are too busy washing their hair (an excuse with which he may at least empathize).

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Moreover, Britain's upcoming referendum on membership in the European Union may be a more significant event in the history of the alliance between the U.S. and the U.K. than the election of Trump. If Britain votes to leave the EU, its political and economic power will be severely diminished. Inevitably, that will mean that the special relationship will matter less to the U.S. (Perhaps predictably, in sharp contrast to the Obama administration, Trump hopes Britain votes for isolation.)