by Matt Kroenig and Dan Negrea

The Wall Street Journal

He understands that the American president can’t draw red lines unless he is willing to enforce them.

On the campaign trail, Donald Trump proudly contrasts the peace that prevailed during his first term with the wars in Europe and the Middle East that erupted during the current administration. Some observers dismiss the former president’s claims and maintain that Joe Biden and Kamala Harris were simply unfortunate to have had conflicts break out on their watch. A closer look shows that Mr. Trump is correct. He instinctively understands the art of deterrence, whereas Mr. Biden and Ms. Harris have consistently, if perhaps unintentionally, undermined this traditional pillar of U.S. strategy.

The Defense Department could be called the Deterrence Department, for the apex of its strategy isn’t to fight wars but to deter them. Successful deterrence requires Washington to convince its adversaries that the costs of attacking the U.S. and its allies greatly outweigh any conceivable benefit. For decades, leading deterrence theorists such as Thomas Schelling and Henry Kissinger argued that a successful posture rests on three Cs: capability, communication and credibility.

First, Washington needs the military capability to impose an unacceptable cost on any adversary. While Mr. Trump invested in U.S. defense capabilities and Mr. Biden and Ms. Harris have cut military spending in real terms, it is unarguable that the U.S. has retained a deterrence capability under both administrations.

The U.S. nuclear arsenal and the capability of American conventional forces ensure that. The difference, therefore, lies in the other two Cs.

U.S. leaders must clearly communicate the consequences that adversaries will suffer if they cross Washington’s red lines. This is one of Mr. Trump’s greatest strengths. He told North Korea’s Kim Jong Un that his threats to the U.S. would “be met with fire, fury and frankly power the likes of which this world has never seen before.”

Mr. Trump told the Journal that he deterred Russia’s Vladimir Putin by saying, “‘Vladimir, if you go after Ukraine, I am going to hit you so hard. … I’m going to hit you right in the middle of fricking Moscow. … I’m going to take those f— domes right off your head.’ Because, you know, he lives under the domes.”

Speaking about Iran’s alleged assassination plots against him, Mr. Trump recommended that Mr. Biden warn that if an adversary kills a former president, “We will bomb that country into oblivion—and it would stop.”

Contrast this with the Biden-Harris communication style. When Mr. Putin amassed forces on Ukraine’s border in February 2022, Mr. Biden didn’t draw red lines. Instead he flashed a green one, proclaiming that the U.S. wouldn’t send troops under any circumstance and that a “minor incursion” might be acceptable. Assured by the American president that there wouldn’t be serious consequences, Mr. Putin invaded shortly thereafter.

Biden-Harris officials have repeatedly stated that avoiding escalation is their foremost priority. But effective deterrence is essentially the threat of escalation held in reserve. By taking that threat off the table, the administration has also removed any possibility for effective deterrence.

The final C is credibility. Do our adversaries believe that if they cross U.S. red lines, the president will follow through on threats and impose serious consequences—or that he will simply back down?

Mr. Trump reinforced red lines, striking Syria in 2017 after Bashar al-Assad used chemical weapons on his own people, and killing Iranian Gen. Qassem Soleimaniin 2020. His reputation for unpredictability kept adversaries guessing. They didn’t know what they could get away with, so they played it safe.

Mr. Biden’s credibility was undermined by the disastrous August 2021 withdrawal from Afghanistan, which sent the message that he didn’t have the stomach for military action. The administration’s reputation for extreme caution means that adversaries know they can push hard without eliciting a response.

When Iran launched a drone-and-missile barrage against Israel in April, Mr. Biden counseled restraint and encouraged Prime Minister Benjamin Netanyahuto “take the win.” The result? Iran learned it wouldn’t suffer any consequences for such behavior and launched another massive attack in October. Deterrence failed again.

Maybe Mr. Biden and Ms. Harris thought they could reassure Americans that their administration wouldn’t start a large war. Maybe they thought America’s adversaries would understand their reasonable intentions and reciprocate in kind. They probably didn’t intend to invite aggression, but that’s exactly what they did and received anyway.

The world can’t risk four more years of a failed American deterrence policy, especially as China threatens to open a third front in the Indo-Pacific with daily military coercion against U.S. allies. The next American president must restore the 3 Cs of deterrence and, with them, global stability and peace. Blessed are the peacemakers.

Mr. Kroenig is a vice president of the Atlantic Council, a professor at Georgetown, and a former Pentagon strategist.

Mr. Negrea is a distinguished fellow at the Atlantic Council and a former senior State Department official.

They are the co-authors of “We Win They Lose: Republican Foreign Policy and the New Cold War.”