• Peter Bergen: Very little attention was devoted to Afghanistan and Syria, two of the biggest challenges facing the new president
• Debate wasted time on whether Trump was for the Iraq War before he was against it, Bergen says

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)The first debate between Hillary Clinton and Donald Trump had plenty of energy and substance when it came to issues of the economy and trade, but it did not have enough substance when it came to the key issue of how the president will act as commander in chief.

Consider that the word "Afghanistan" went unmentioned, except for Clinton briefly citing the war as an example of NATO cooperation. This is a strange oversight as America's longest war has ground on for a decade and half, while the Taliban are now estimated to control some 30% of the country.

One of the first things the next president will have to decide is whether to continue to draw down troops in Afghanistan, as the Obama administration had planned, to effectively zero, or whether conditions on the ground merit a substantial long-term presence of US troops in noncombat roles helping to support the Afghan army and police.

After all, no one wants to see what happened in Iraq after the withdrawal of US forces in 2011 and the rise there three years later of ISIS. Already, ISIS has a foothold in Afghanistan.

Also unexplored was the issue of immigration. One of Trump signature ideas is the banning of Muslim immigration, which lately has morphed into the banning of immigration from countries that have terrorism problems.

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Does that include France, which has had a number of terrorist attacks since 2014? And how would banning immigrants from countries experiencing terrorism change the fact that every lethal terror attack in the States since 9/11 has been carried out by an American citizen or legal permanent resident? Indeed, the first terrorist attack in Manhattan since 9/11 that injured 31, which occurred earlier this month, was carried out by an American citizen.

Trump's ideas around banning certain kinds of immigrants were not discussed in the debate.

Syria, which is now the worst humanitarian crisis arguably since World War II, with more than 400,000 dead and some 11 million refugees, also was barely mentioned. This is especially surprising, as during the past several days the major city of Aleppo is being compared to Dresden under the Nazis, so great is the damage being inflicted on the city by Syrian and Russian warplanes.

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Clinton discussed her plans to take out ISIS, including additional airstrikes; more resources devoted to fighting ISIS' online presence; more help to the Kurdish forces fighting ISIS, and more effort to take out ISIS' leaders, all of which are sound ideas, but differ little from what the Obama administration is already doing.

Many precious minutes in the debate were spent arguing over whether Trump was for the Iraq War before he was against it, as the factual record shows. Yet even if Trump was, like Clinton, once a supporter of President George W. Bush's Iraq misadventure, that data point doesn't shed any light on what to do now about the threat of ISIS.

One of the key foreign policy differences between President Obama and Clinton is her call for a no-fly zone in northern Syria, which would go a long way to reduce the casualties on the ground and to stem the massive tidal wave of refugees pouring into the Middle East and Europe.

Of late, Clinton has not mentioned the no-fly zone much, but it's long overdue and there surely must be a way to enforce it that will not start a war with Russia. After all, under President Bill Clinton, the US enforced a no-fly zone over Iraq for many years without losing a single American plane.

Bizarrely, Trump cast doubt that the hack in June on the Democratic National Committee was the work of the Russians, despite the fact that every US government official with knowledge of the matter points the finger at them, as do independent cybersecurity groups.

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Trump and Clinton had a rare moment of agreement that suspected terrorists on the "no-fly" list shouldn't be allowed to legally buy guns. Congress failed to pass this common-sense measure after the terrorist attack in Orlando in June in which 49 people were killed. The next commander in chief should push for this. After all, if you are deemed to be too dangerous to be allowed on a plane, it makes no sense that it's OK for you to buy a gun.

Hopefully, in the next two presidential debates we will hear more from both candidates about what comes next in Afghanistan, about what steps could tamp down the Syrian civil war, about how to stabilize Iraq after ISIS is largely defeated and about Trump's anti-terrorism plans.