Even Trump supporters are bemoaning the fact that the Donald refused to prep sufficiently for the first presidential debate.

But his lapse was more than a display of intellectual laziness. It goes to the heart of why a President Trump would endanger American security and why he must never become America’s commander-in-chief.

As was obvious on Monday, the GOP candidate has no grasp of what’s required to be a global leader. He still acts like a reality TV star who’s using the media and twittersphere to stoke populist anger and sell the Trump brand.

He’s said he consults himself on foreign policy, and he is resistant to conferring with experts. He openly scoffed at Hillary Clinton for spending several days on debate preparation. “Yes, I did.” Clinton said, with a wry smile. “And you know what else I prepared for? I prepared to be president. And I think that’s a good thing.”

She’s damn right.

But Trump defied his campaign manager’s pleas to bone up before the debate, preferring to shoot the breeze with Rudy Giuliani. This is a man who refuses to learn, grow or read. The world takes notice of his careless language.

There were a host of troubling foreign policy Trumpisms at the debate, but here are three that are especially troubling. They will unnerve our allies and amuse our opponents.

Trump could care less.

• One: Trump doubled down on his embrace of Vladimir Putin. This becomes harder to understand as Putin’s behavior grows ever more ugly.

The issue of cybersecurity was raised, including the hacking into the Democratic National Committee. U.S. intelligence agencies and leading private U.S. cyber security firms are convinced the hacks were initiated by Russian intelligence agencies.

Trump (who has urged Putin to hack Clinton’s emails) rose to Russia’s defense. “I don’t think that anybody knows it was Russia that broke into the DNC,” he insisted. “It also could be somebody sitting on their bed that weighs 400 pounds.”

Why raise this idiotic conjecture when strong evidence points to the Kremlin? Why whitewash Putin when the Russian leader is threatening our European allies and committing war crimes in Syria? Does Trump’s dream of real estate deals in Moscow blind him to Moscow’s deeds?

This week, a Dutch-led investigation concluded that the surface-to-air missile system used to shoot down a Malaysia Airlines civilian plane over Ukraine in 2014 came from Russia.

Last week, Russian aircraft (or their Syrian allies bombed a U.N. humanitarian convoy entering Aleppo. There was no “fog of war”; the convoy’s arrival had been cleared with the Syrian government and the convoy was clearly marked. This war crime was part of a deliberate Syrian-Russian strategy to conquer eastern Aleppo, including the bombing of the last hospitals and the massacre of 1,000 civilians over the past week.

Yet Trump slobbers over Putin, convincing the Kremlin and our European allies that, as president, he would give the Russian leader a free pass.

• Two: Trump again defamed our allies. Once again he suggested he might abandon our NATO and Asian allies because they weren’t paying their fair share of defense costs. “We defend Japan, we defend Germany, we defend South Korea, we defend Saudi Arabia … They do not pay us,” said the Donald. Wrong. Wrong. Wrong.

Japan and South Korea pay billions of dollars to cover much of the basing and construction costs. As for Saudi Arabia, let’s face it, we are protecting Saudi oil in our own interest (and the Saudis buy billions worth of U.S. arms).

Germany pays some, but should pay more; however, most experts believe it would be more expensive to move and base Germany-based troops stateside.

More to the point, Trump seems blind to the strategic importance of alliances with countries that share our democratic values, at a time when authoritarian Russia and China are rising and terrorists threatening. “When you look at NATO … I’m a business person,” he insists, with his typical tunnel vision.

Our alliances are about more than dollars and cents.

The reality TV showman just doesn’t get how U.S. influence would shrink if we broke our commitments to allies. He doesn’t grasp how he undermines those allies by questioning our commitments at a time when Russia is expanding.

“Words matter,” Clinton rightly said at the debate. “Words matter when you run for president. And they really matter when you are president. I want to reassure our allies in Japan and South Korea and elsewhere that we have mutual defense treaties and we will honor them.”

About such realities, the Donald doesn’t have a clue.

• Three: Trump repeated his mantra: “we should have taken the oil” in Iraq. Never mind that Iraq is a sovereign country, and America’s seizure of its oil would be the equivalent of Saddam Hussein’s 1990 seizure of Kuwait’s fields. Never mind that the whole world would have opposed this. And it would have confirmed the terrorist mantra that the United States invaded Iraq to take its oil.

Trump doesn’t see the big picture. To him foreign policy is only a business balance sheet. He’s always willing to declare bankruptcy and let the contractors go hang.

Trump defenders argue he will learn or surround himself with geniuses, but as the debate made clear Trump thinks he’s the genius and nothing more is needed. Heaven help our republic if a majority of voters agrees.

— Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer.