Republican presidential nominee Donald Trump speaks during the presidential debate at Hofstra University in Hempstead, N.Y. (Patrick Semansky/Associated Press)

When nuclear weapons policy came up in Monday’s presidential debate, Donald Trump showed once again that he doesn’t have basic knowledge of the details and hasn’t thought through how he would handle America’s nuclear arsenal if elected. It’s only the latest example of his refusal to study up on national security.

Hillary Clinton first brought up the subject of nuclear weapons Monday as an argument that Trump doesn’t have the temperament to be commander in chief. She referred to Trump’s previous statements endorsing the idea that allies including Japan, South Korea and Saudi Arabia might get their own nuclear weapons.

“He even said, well, you know, if there were nuclear war in East Asia, well, you know, that’s fine, have a good time folks,” she said.

“Wrong.” Trump responded. “Nuclear is the single greatest threat.”

Moderator Lester Holt followed up with a new question about nuclear policy neither Trump nor Clinton had faced before. He referred to my own reporting that President Obama is considering changing longstanding U.S. nuclear policy by declaring a “No First Use” policy, meaning the United States would promise never to use nukes first in any conflict.

“I would like everybody to end it, just get rid of it. But I would certainly not do first strike. I think that once the nuclear alternative happens, it’s over,” Trump began, appearing to endorse the proposed policy change. But then he contradicted himself.

“At the same time, we have to be prepared. I can’t take anything off the table,” he said.

There’s no way that Trump as president could have a policy that promises not to strike first with nuclear weapons and also keeps the option of a nuclear first strike on the table. Experts said Trump simply didn’t know what he was talking about.

“Trump appears not to understand the important nuances of nuclear strategy,” said Daryl Kimball, executive director of the Arms Control Association, which supports a “No First Use” nuclear policy. “He wants to have it both ways. He wants to seem as if he would exercise restraint but he wants to project fear in our adversaries that he might use nuclear weapons. It reflects a lack of understanding that the commander in chief needs mastery of.”

In his answer, Trump also said America’s B-52 bombers are too old, that Russia’s nuclear weapons are newer than ours, that the Iran deal should have included a requirement that Iran solve North Korea’s nuclear threat and at the same time that China should solve the North Korean nuclear threat.

“China should solve that problem for us,” said Trump. “China should go into North Korea.”

Clinton, in her answer to the question about “No First Use” nuclear policy, didn’t say she was for or against it. But at least she didn’t say she was for AND against it. She called for America to stand with allies and honor its defense commitments. As I’ve also reported, allies including Japan and South Korea have told the Obama administration they are opposed to the United States declaring a “No First Use” nuclear policy.

Trump’s stumbles on nuclear policy at the debate come after several other instances where he appeared not to understand the basics of the issue he says is the number one threat to the nation. He has suggested he might use nukes against the Islamic State. He often says he would be “unpredictable” about how and when he might use nuclear weapons. In a primary debate, he had no idea what the “nuclear triad” was.

Trump’s lack of basic understanding about nuclear weapons policy is concerning enough. But even more troubling is Trump’s lack of progress over the past few months on national security issues overall. He simply refuses to make the effort to prepare himself for the job of commander in chief.