This story has been updated.
Syria has been embroiled in a violent, bloody war for several years that has forced millions of people to flee and seek safety in surrounding countries and in Europe. The United States has recently come under fire for not accepting as many refugees as several other major nations.
But Trump raised questions about these refugees and their intentions.
"Did you ever see a migration like that?" he said. "They're all men, and they're all strong-looking guys... There are so many men; there aren't that many women. And I'm saying to myself: Why aren't they fighting to save Syria? Why are they migrating all over Europe? Seriously."
Trump floated an alternative theory, suggesting that these refugees are perhaps a terrorist army in hiding. He said there are proposals to accept as many as 200,000 refugees in coming years, a number he called "inconceivable." These refugees have "no identification," he said, and it's unknown where exactly they are from.
"Military tactics are very interesting," Trump said. "This could be one of the great tactical ploys of all time. A 200,000 man army, maybe. Or if they sent 50,000 or 80,000 or 100,000... That could be possible. I don't know that it is, but it could be possible."
Trump said that if he is elected president, he will force all Syrian refugees to leave the United States. That promise prompted boisterous applause -- the loudest he received that night.
"They're going back," Trump said. "And if I lose, I guess they're staying. But if I win, they're going back."
At Jeb Bush's New Hampshire town hall Wednesday evening, a Syrian-American woman teared up as she asked the former Florida governor how he would address the refugee crisis, and talked of family members who had fled the country. After the event, Bush sharply criticized Trump's remark.
"Send them all back? To a hellhole?" Bush told reporters. "This is the same guy, by the way, that is also advocating exactly what seems to be supportive of Putin and his emergence in Syria. That's not the proper policy for the United States."
Sean Sullivan contributed to this story.