The main challenge for Donald Trump in his first debate with Hillary Clinton this week was always clear: He needed to pass the commander in chief test. And he failed. Voters saw two candidates on the stage Monday evening: one who had done her homework and thought seriously about questions of war and peace, and another who, to all appearances, had neither cracked a book nor listened carefully at a single foreign-policy briefing.

Both candidates agreed that cyberwarfare — the growing efforts by Russia, China and others to hack American computer networks — are a top priority for the next president. “It is a huge problem,” Trump affirmed.

His policy, then? “We have to get very, very tough,” he said. “I have a son. He’s 10 years old. He has computers. He is so good with these computers, it’s unbelievable. The security aspect of cyber is very, very tough. And maybe it’s hardly doable. But I will say we are not doing the job we should be doing.”

When candidates cite their children as experts, it’s a safe bet they haven’t studied the issue themselves.

Nuclear weapons, then; surely Trump has thought hard about the most powerful part of the U.S. military arsenal.

It’s been widely reported that President Barack Obama has considered adopting a policy of “no first use,” meaning the United States would promise never to use nuclear weapons first in any conflict. It’s not a new or obscure issue.

Trump’s view? “I’d like everybody to end it, just get rid of it, but I would certainly not do first strike. I think once the nuclear alternative happens, it’s over,” he said. “At the same time, we have to be prepared. I can’t take anything off the table.”

Oops, as a previous debater once said. In his answer, Trump took two opposing positions at the same time. “I would certainly not do first strike,” he said — that’s “no first use.” But “I can’t take anything off the table” — that’s the opposite of “no first use.”

It sounded as if he didn’t know what he was talking about.

But not all potential conflicts are nuclear. Earlier this month, after Iranian gunboats harassed U.S. Navy vessels in the Persian Gulf, Trump issued a warning to Tehran:

“When they circle our beautiful destroyers with their little boats, and they make gestures at our people that they shouldn’t be allowed to make, they will be shot out of the water,” he declared.

At the debate, Clinton said that kind of hair-trigger reaction could start a war.

Trump didn’t back down. “That would not start a war,” he said, without saying why. “They were taunting us,” he added.

Last example: To keep peace around the world — and wage war, too — the United States has long relied on military alliances with Japan, South Korea and the mostly European members of the North Atlantic Treaty Alliance.

“I haven’t given lots of thought to NATO,” Trump admitted on Monday, before sounding off anyway. “We pay approximately 73 percent of the cost of NATO,” he claimed. (He’s had that number wrong all year. The U.S. pays about 22 percent of NATO’s direct costs.)

“We’re defending them, and they should at least be paying us what they’re supposed to be paying by treaty and contract,” he said. “We defend Japan, we defend Germany, we defend South Korea, we defend Saudi Arabia, we defend countries. They do not pay us, but they should be paying us, because we are providing tremendous service and we’re losing a fortune.”

There’s certainly an argument that U.S. allies should spend more money on defense, including higher subsidies for U.S. bases in their countries. But do we really want to convert mutual defense treaties into contract-for-service agreements? There’s no sign that Trump has spent even a minute weighing the consequences of such a shift.

“Words matter when you run for president,” Clinton responded. “I want to reassure our allies in Japan and South Korea and elsewhere that we have mutual defense treaties and we will honor them.”

Trump has every right to criticize Clinton’s foreign policy positions, of course. Her support for Obama’s nuclear deal with Iran, her decisions in Libya and her fruitless efforts to increase aid to rebels in Syria are all fair game.

But a presidential candidate can’t just offer critiques; he has a duty to offer coherent alternatives, too. Trump hasn’t.

Last year, when Trump was merely a long-shot candidate for the Republican nomination, he had a semi-plausible excuse: As a businessman, he couldn’t be expected to know as much as a former secretary of state.

But he’s been a de facto nominee for six months. In less than six weeks, he could be president-elect of the United States. He’s had plenty of time to learn. He’s had access to many of his party’s experts, retired military officers and even CIA briefers who might be glad to fill him in on the nuances of world affairs.

Instead, he continues to recycle the bromides he once said he learned by watching “the shows.” He appears to have studied nothing and learned nothing.

That alone should disqualify him from the presidency.

Doyle McManus is a columnist for the Los Angeles Times. Readers may send him email at doyle.mcmanus@latimes.com