Story highlights

• Peter Bergen: Trump fell back on a familiar mélange of empty slogans and false claims to describe what he'd do
• Bergen: Clinton, by contrast, had some sensible policy proposals, including a no-fly zone over northern Syria

Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of "United States of Jihad: Investigating America's Homegrown Terrorists." The opinions expressed in this commentary are his.

(CNN)Since the U.S. president as commander in chief controls the world's most powerful military, it was sobering to watch Donald Trump and Hillary Clinton debate national security issues on Sunday night.

Trump fell back on a familiar mélange of empty slogans and false claims (a.k.a. lies) to describe what he would do as the nation's next leader. He demonstrated he is completely unqualified for the supremely difficult job of commander in chief.

Trump said "I will knock the hell out of ISIS," seemingly unaware that 45,000 ISIS fighters have been killed in the past couple of years by the US-led coalition, according to US Army Lt. General Sean McFarland, who was in charge of that coalition until two months ago.

As a result, there are only an estimated 19,000 to 25,000 ISIS fighters left, while ISIS' recruitment of "foreign fighters" has plunged from an estimated 2,000 a month to a trickle of only 50 a month.

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ISIS now has a serious math problem because the attrition rate from US air strikes is vastly more than the group's capacity to attract new fighters. This may be why ISIS propaganda is increasingly featuring child soldiers; the group is simply running out of men.

Attacking Mosul

When it came to a discussion of the impending coalition attack on Mosul, the ISIS-held city that is the second largest in Iraq, Trump said at Sunday's debate, "We have announcements coming out of Washington and coming out of Iraq, we will be attacking Mosul in three weeks or four weeks. Well, all of these bad leaders from ISIS are leaving Mosul. Why can't they do it quietly? Why can't they do the attack, make it a sneak attack?"

Debate co-moderator Martha Raddatz of ABC News, who has spent considerable time in the Iraqi war zone, gently suggested that the U.S. military might have an actual strategy in making these announcements, proffering that they could be "psychological warfare."

Trump, who has previously said he knows more about ISIS than the generals, said he couldn't think of any military reason for these announcements saying, "I can't think of any. And I'm pretty good at it." This is a bizarre claim from someone who repeatedly avoided service in Vietnam and has no record of visiting any of America's post-9/11 war zones.

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Raddatz proffered another reason for why the US military was signaling an impending attack on Mosul, saying, "It might be to help get civilians out." More than a million refugees may pour out of Mosul as the battle there intensifies. But Trump seems to have not considered that the US military may have a plan with these announcements about the Mosul campaign.

Raddatz followed up by asking Trump what his strategy in Iraq is. Trump had no answer, instead falling back on the claim that "200 admirals and generals" backed him. In fact, 88 retired generals and admirals endorsed Trump last month. Clinton has the backing of 110 retired generals and admirals.

Muslims and refugees

At Sunday's debate Trump also said Muslims should "come in and report when they see something going on. When they see hatred going on, they have to report it."

Muslims are, in fact, already doing precisely that. According to research by New America, of the more than 360 jihadist terrorism cases since 9/11 in the US, more than a quarter were precipitated by tips from the Muslim community or from family members.

Trump repeated a thoroughly debunked claim about the San Bernardino terror attack in December, saying "many people saw the bombs all over the apartment of the two people that killed 14 and wounded many, many people." There is no evidence for this assertion.

When asked about one of his signature plans, a ban on Muslim immigration, Trump said that the plan had "morphed into a extreme vetting from certain areas of the world." Trump claimed that there are "hundreds of thousands of people coming in from Syria when we know nothing about them. We know nothing about their values and we know nothing about their love for our country."

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This is another false claim. The United States has only taken a paltry 10,000 Syrian refugees, despite the fact that Syria is now in the grip of one of the worst humanitarian disasters since World War II. Almost five million refugees have fled the country and another six million have fled their homes but remain inside Syria.

And refugees are not a big source of terrorism in the United States. According to New America's research, of the 360 cases of jihadist terrorism in the US, refugees committed only 12.

Contrast that with the fact that every lethal jihadist terrorist crime in the US since September 11, 2001, was committed by an American citizen or legal permanent resident. And 81% of the 360 jihadist terrorism cases since 9/11 have involved American citizens or legal residents.

Trump also continued his strange campaign to defend Putin's Russia, saying we don't know that the hacking of the Democratic National Committee (DNC) emails was the work of Russians. This came only two days after the U.S. Director of National Intelligence, James Clapper, released an official statement saying that the DNC hacks "are intended to interfere with the U.S. election process" and "that only Russia's senior-most officials could have authorized these activities."

Does Trump have some special intel about the perpetrators of the hacks that the US intelligence community is unaware of? Of course not.

Clinton's proposals

Hillary Clinton, by contrast, had some sensible policy proposals during the debate, including one she has proposed in the past: A no-fly zone over northern Syria that would help to prevent the wholesale slaughter of Syrians by the Assad regime's warplanes in Aleppo, which is now being compared to Dresden, Germany during World War II.

In recent months Clinton has been silent on the no-fly zone issue, but the collapse of the recent peace initiative with the Russians seems to have emboldened her to push this plan again. Clinton also endorsed the effort to investigate war crimes "committed by the Syrians and the Russians."

She also claimed she "would not use American ground forces in Syria," which is something of a distinction without a difference since US Special forces are fighting in Syria already.

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Clinton said she would "go after Baghdadi," the leader of ISIS. This is surely what the Obama administration is already doing, given the fact that US airstrikes have killed more than 135 ISIS leaders and officials.

To summarize, Trump has no serious plan for the key foreign policy problem facing the US today -- the implosion of Syria. Clinton would continue the Obama strategy, which has certainly not brought peace to Syria, but has imposed significant costs on ISIS. She would also impose a no-fly zone, which would tamp down the slaughter of civilians and reduce the refugee flow out of Syria.

Unanswered questions

In the Clinton-Trump debate, there was no discussion at all of America's longest war in Afghanistan, where the Taliban now controls a third of the country. What should the next commander in chief do there?

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There was also no discussion of what happens when Mosul falls. How do the US and its allies plan to govern the city and encourage a political accommodation in Iraq that can reduce the sectarian tensions that produced ISIS in the first place?

And as the campaign against ISIS reaches a climax, what of the thousands of Western foreign fighters in Iraq who are not captured or killed on the battlefield? Where will they go?

Hopefully we will get some answers to these questions in the final debate.