IN NO realm is the U.S. president more powerful than in matters of national security. A president may launch wars, conduct intelligence operations and strike deals with foreign governments with minimal checks from the courts and Congress. If he wished, a President Donald Trump could carry out — or at least order — many of the most extreme proposals he has tossed off during the election campaign.

If Mr. Trump chose to “take the oil” of Iraq or Syria, as he has frequently proposed, he could direct the armed forces to plan and launch such an operation, whatever its prospects for success. If he wanted to assassinate foreigners who opposed him, he could unilaterally and secretly change the executive order prohibiting such action. If he wanted to reinstate waterboarding and other torture methods for use on detainees, he could cancel President Obama’s orders governing torture and appoint lawyers to the Justice Department’s Office of Legal Counsel willing to issue a secret opinion invalidating a congressional ban.

Mr. Trump could launch surveillance programs targeting foreigners without informing Congress. He could scrap — again, without public notice — the rules Mr. Obama instituted for limiting U.S. drone strikes. He could shut U.S. bases abroad, withdrawing troops from Japan, South Korea and Europe.

Because the GOP candidate has evidently done no serious thinking about national security, which of these actions he might take is necessarily a matter of guesswork. Yet two aspects of Mr. Trump’s worldview appear deeply rooted and consistent over a number of years. One is his disregard for traditional U.S. alliances, from Mexico to NATO to Saudi Arabia and Japan. The other is a strong and somewhat mysterious attachment to Russia’s Vladi­mir Putin. If Mr. Trump were to act on those instincts, he could transform the world, very much for the worse.

It would not be necessary for Mr. Trump to take any formal action to wreck NATO, which has been the foundation of Western security since 1949. If he merely repeats, as president, the suggestion that the United States might not defend states in Central and Eastern Europe that are threatened by Russia, it will send them scrambling to make other arrangements — including their own deals with Moscow.

On Sept. 7, during a town hall event hosted by MSNBC, Republican presidential candidate Donald Trump stated he would have “a good relationship” with Russian president Vladimir Putin. It wasn’t the first time he's been friendly towards him. (Adriana Usero/The Washington Post)

Mr. Putin’s fondest wish is for a U.S. president to recognize Eurasia as a 19th-century-style Russian sphere of influence. Presidents Obama and George W. Bush scorned that notion, but Mr. Trump seems likely to embrace it. Whether that is because Mr. Trump genuinely believes that Ukraine and its neighbors deserve to be dominated by the Kremlin, or because of his own financial relationships with Russian banks and oligarchs, is uncertain, due to Mr. Trump’s refusal to release business records and tax returns.

In Asia, a Trump election could prompt Japan and South Korea to reconsider whether to produce their own nuclear weapons — in part because Mr. Trump himself has suggested it. The same is true of Saudi Arabia, which the GOP candidate has been describing since the 1980s as unworthy of U.S. defense. Dictators across the world, from Thailand to Syria to Egypt, would cheer the arrival of an American president who would excuse, and maybe even celebrate, their human rights crimes.

In sum, the election of Mr. Trump would likely bring about the end of the era of American global leadership that began in 1945. The U.S. alliances built after World War II, which have been the foundation of that strength, would be disregarded. A new, cynical, self-interested America would emerge, ready to use walls, boycotts, assassination and torture to achieve its aims, and to partner with like-minded regimes such as Russia. For those who believe in traditional American liberal values, the world would become a much colder — and more dangerous — place.