Republican presidential candidate Donald Trump and Democratic candidate Hillary Clinton during their first presidential debate Monday. (LEFT: Paul J. Richards/AFP/Getty Images; RIGHT: David Goldman/AP)

The first presidential debate between Hillary Clinton and Donald Trump covered a lot of ground Monday night. But despite a 30-minute portion entirely devoted to the subject “Securing America,” there were plenty of issues related to the defense and national security of the United States that did not come up at all.

Here are five:

The future of Afghanistan
Next month, the United States will mark the 15th anniversary of military operations in the country where the Sept. 11, 2001, attacks were planned. But despite the deaths of more than 2,200 U.S. troops there and the wounding of many more, the country remains mired in conflict.

Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, told the Senate Armed Services Committee last week that 10 percent of Afghanistan is now under the control of the Taliban, and an additional 20 percent is contested by it. Dunford described the conflict between Afghan security forces and insurgents as “roughly a stalemate.”

Taliban leaders watched the debate Monday night, a spokesman told NBC News. Like Americans watching the debate, they didn’t hear anything about how the next commander in chief will handle America’s longest-running war.

The South China Sea feud
China’s rise as a nation came up several times during the debate, mostly through the lens of how it affects U.S. trade. It also came up when both Clinton and Trump addressed the recent hack of the Democratic National Committee. Clinton said that Washington has to make it clear that it has a greater capacity to carry out such operations, “whether it’s Russia, China, Iran or anybody else” and Trump said that it could have been carried out Russia, China or “somebody sitting on their bed that weighs 400 pounds.”

But the simmering feud in the South China Sea — and China’s military expansion there — was not mentioned at all. Several countries have made claims to both maritime passageways and islands, and the United States has sought to avoid taking a side while maintaining freedom of navigation through international waterways.

Earlier this year, the United States and the Philippines, a treaty partner, announced that they had signed a deal that allows conventional U.S. forces to deploy to the Philippines for the first time in decades. But the U.S. relationship has grown increasingly complex this spring since the election of President Rodrigo Duterte, who has cursed out President Obama and raised the possibility that U.S. Special Operations troops may be forced to leave the southern Philippines after years of working with Philippine troops.

How to put Iraq back together
Clinton and Trump shared a lively exchange about how to handle the Islamic State, also known as ISIS, in which Trump accused her of “telling the enemy everything you want to do” and Clinton countered that he has no plan whatsoever. But the candidates did not once mention the Islamic State-held city of Mosul, the coming battle in which U.S.-backed Iraqi forces will attempt to take it back, and how the international community will assist in reconstruction.

Clinton did say that the United States must intensify airstrikes against the militants and assist “Arab and Kurdish partners to be able to actually take out ISIS in Raqqa,” the Islamic State’s de facto capital in Syria. But the battle for Mosul and surrounding areas is expected to come first, and could bring a major humanitarian crisis. In fact, Defense Secretary Ashton B. Carter said in July that putting Mosul back together is the “biggest strategic concern” among defense ministers he has met with.

What to do with America’s aging nuclear arsenal
Trump and Clinton agreed that nuclear weapons are the top threat the world faces. But they didn’t address a complicated, related issue: What to do with the aging, Cold War-era arsenal of weapons the United States maintains.

The arsenal includes three options known as the “nuclear triad” — bombs delivered by ballistic missile, aircraft or submarine — all of which are aging. Trump at one point mentioned that the B-52 bomber is old enough that “your grandfather could be flying them,” but neither candidate broached how Washington should pay for upgrades that could collectively cost up to $1 trillion, according to nuclear weapons analysts.

On Monday afternoon, Carter visited Minot Air Force Base in North Dakota and talked about the U.S. plan to rebuild its ballistic missile arsenal, saying that as long as other nations have nuclear weapons, the United States must maintain them to discourage their use — a policy known as nuclear deterrence.

“I do not think that as long as nuclear weapons exist that there is a replacement for nuclear deterrence,” Carter said. “That’s the foundation of our view, and the president’s view, which I cited, that as long as nuclear weapons exist, the United States needs to have a safe, secure and reliable deterrent.”

Veterans
After 15 years of war, there are more than 2.5 million U.S. veterans of the wars in Iraq and Afghanistan. But neither moderator Lester Holt nor Clinton and Trump mentioned them once Monday night, prompting criticism afterward from the nonprofit group Iraq and Afghanistan Veterans of America (IAVA).

“Veterans issues are national security issues,” IAVA chief executive Paul Rieckhoff said in a statement released after the debate. “Both candidates missed an incredible opportunity tonight to devote at least some portion of their time to focus on our nation’s veterans.”

Rieckhoff said reforming Veterans Affairs, protecting benefits promised to veterans in the GI Bill, improving care for female veterans and focusing on the treatment of post-traumatic stress and suicide all were “notably absent from a discussion segment focused on national security.” Some of those issues came up in a forum IAVA hosted with NBC News earlier this month, but the format then did not allow candidates to debate them.

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