It is best to be both feared and loved, Machiavelli said, but if one must choose, it is safer to be feared. His reason: Friendships acquired at a “price” rather than with “greatness and nobility of spirit” are unreliable when the going gets tough.

Donald Trump understands this. “Real power is—I don’t even want to use the word—fear,” he told Bob Woodward and Robert Costa in a 2016 interview.

Creating fear is his go-to strategy for inducing people to comply with his wishes. If threats don’t suffice, he moves against vulnerable individuals and institutions, making examples of them to terrify others into obedience.

In the early months of Mr. Trump’s second term, this strategy has proved remarkably successful. His threats to pull federal funding from Columbia University caused several well-endowed universities to capitulate. His threats to strip law firms whose clients have opposed his policies of security clearances, access to federal buildings and government contracts have sent some of these firms’ leaders scurrying to accept his demands. The MAGA crowd has successfully intimidated Republican lawmakers who oppose Mr. Trump’s policies or nominees with threats of primaries unless they fall in line. Fearing total loss of influence with the administration, business leaders and government officials who loathe Mr. Trump’s tariffs are biting their tongues.

But there are limits to the effectiveness of fear as a tactic. Although some smaller countries such as Vietnam are offering concessions to ward off the president’s tariffs, others—including giants such as China and Canada—have already responded with countermeasures, and the European Union also is preparing a firm response.

Resistance to threats often reflects pride as well as material interests. Mr. Trump’s warning that the U.S. will acquire Greenland “one way or another” hasn’t intimidated Denmark, whose population numbers only six million. His insistence that Canada must become the “51st state” has provoked a wave of angry nationalism among Canadians across the political spectrum.

Resistance becomes more likely when the people and institutions under threat decide that the costs of compliance, measured in moral and material terms, exceed the benefits. Larry Summers—an economist and former official in the Clinton and Obama administrations who was Harvard University’s president from 2001-2006—urged Harvard in a recent op-ed to use a portion of its $50 billion endowment if needed to offset the damage from the Trump administration’s threatened funding cuts. If Harvard, with its huge financial resources, reputation and influential alumni network can’t resist the “arbitrary application of government power, who else can?” Mr. Summers asks. “Without acts of resistance, what protects the rule of law?” He doesn’t pretend that universities have been faultless in handling antisemitism, the excesses of identity politics and limits on free speech. But he insists that these errors don’t justify the Trump administration’s politically motivated threats—or universities’ capitulation to them.

When fear loses its power, the reaction of its victims can be harsh: Witness the fate of dictators such as Mussolini, Gadhafi, and Ceausescu at the hands of their angry countrymen. If Mr. Trump’s tariffs end up raising prices, slowing growth and increasing unemployment, Republicans who have gone along will likely break ranks, with expressions of resentment at having been pressured into supporting policies they don’t believe in.

Elon Musk ’s leadership of the Department of Government Efficiency has reportedly appalled many members of the tech community, but many are too terrified to say so. The sight of industry leaders lined up behind Messrs. Musk and Trump has intimidated them into silence.

But fear is a double-edged sword. In his book “The Prince,” Machiavelli told the story of the Duke Valentino , who conquered Romagna but found it nearly ungovernable. He therefore appointed Remirro de Orco, a “cruel and ready man,” to whom he gave the “fullest power.” Remirro soon quelled disorder, but with tactics that made him widely hated. Desiring to show the people that these excesses stemmed from the “harsh nature of his minister” rather than himself, the duke had Remirro cut in half and displayed in a public plaza.

Politico reports that Mr. Trump has told his close associates that the increasingly unpopular Elon Musk—who has criticized Peter Navarro , the president’s senior trade adviser, and has endorsed free-trade arguments—will be leaving the administration soon. At least Mr. Musk will escape with his body intact, if not his fortune and his reputation. When he returns full-time to the private sector, he can expect a less-than-friendly reception from many corporate leaders.

For similar reasons, President Trump should fear the blowback from a governing strategy largely built on fear.