Who would have thought the displacement of millions of Syrian refugees from their homes by violence and war would come down to an argument over a bowl of Skittles? Yet that's where the 2016 campaign for the White House landed this week after the son of the GOP presidential nominee, Donald Trump Jr., tweeted a flippant missive comparing starving, desperate people to poisoned candies. “If I had a bowl of Skittles and I told you just three would kill you, would you take a handful?” the message asked. “That's our Syrian refugee problem,” it said.

No, that is not the problem — as Wrigley, the company that makes Skittles, was quick to point out: “Skittles are candy,” the company said in a statement. “Refugees are human beings.” Trump Junior may have thought he was being clever, but the joke backfired when the internet lit up with critics condemning his comment's seeming obliviousness to the plight of anyone outside the world of wealth and privilege he inhabits.

He is, of course, also completely wrong. This month, the Cato Institute calculated that “the chance of an American being killed in a terrorist attack caused by a refugee is 1 in 3.64 billion per year.” By the Washington Post's math, you'd need a bowl of Skittles the size of 1.5 Olympic swimming pools for the younger Mr. Trump's analogy to hold up.

What's ironic about the return of the debate about the safety of Syrian refugees is that it comes at a time when we have been confronted by the kind of actual risks we face in the form of last week's bombings in New York and New Jersey. And unfortunately, neither candidate has a good answer for attacks like those.

Mr. Trump has said he would keep Americans safe by imposing a temporary ban on all Muslim immigration to the country and striking hard at overseas terrorist groups like the Islamic State. Ms. Clinton, by contrast, says she would wage an aggressive propaganda campaign for the hearts and minds of Muslim youth to keep them from being radicalized by violent extremists. She would also set up the equivalent of neighborhood watch groups to monitor people who undergo sudden personality or behavioral changes suggesting they might be susceptible to religiously inspired violence and to report them to authorities.

Quite aside from the legal and moral issues raised by Mr. Trump's proposal to halt Muslim immigration to the U.S., however, such a policy clearly would not have stopped Ahmad Rahami, the 28-year-old New Jersey man of Afghan descent charged with planting bombs there and in New York's City's Chelsea district. Mr. Rahami had been in the United States for years and apparently was already a naturalized citizen when he became radicalized.

Similarly, Ms. Clinton's plans to keep people already in the U.S. from being radicalized and to ask their families, friends, neighbors and religious leaders to report suspicious behavior won't work if potential terrorists hide their intentions, as Mr. Rahami apparently did. Some of the people around him apparently did notice that he recently had become more devout in his religious observances after traveling abroad, but that hardly equates to evidence of terrorist inclinations. Moreover, immigrants who have come to the U.S. fleeing brutal authoritarian regimes that maintain power through legions of secret police informers may be reluctant to embrace what looks like a similar form of repression here.

Unfortunately, there's probably no way to completely protect ourselves against another terrorist attack, only more or less practical measures to reduce that risk. Most Americans already suspect that an attack like the ones in New Jersey and New York could happen again. That's why we hope that when that subject comes up during Monday's presidential debate, Mr. Trump and Ms. Clinton will both face up to that harsh reality and level with the American people about what they can and can't be expected to accomplish — and avoid red herrings like the Syrian refugees.