Top Republicans have invoked a centuries-old moral doctrine to defend the ongoing war against Iran.

Last week, Vice President JD Vance and House Speaker Mike Johnson — first and second in the presidential line of succession — explicitly cited “just war” theory while speaking about the Middle East conflict.

They wielded the theory as a cudgel to castigate Pope Leo, whose staunch opposition to the weekslong war has ignited a high-profile clash between the Vatican and the White House.

But, the Republican leaders’ application of the doctrine — which dates back over a thousand years — is fundamentally flawed, according to experts in theology, philosophy and military strategy.

“You can apply the Just War principles to this conflict — and the current U.S. operation fails against every single criteria,” David Whetham, a professor of ethics and the military profession at King’s College London, told The Independent.

‘Americans who liberated Holocaust camps’

During a Turning Point USA event in Georgia last Tuesday, Vance took issue with the pope’s sweeping denunciation of armed conflict.

On April 10, Leo, the first U.S.-born pontiff, wrote on X: “God does not bless any conflict. Anyone who is a disciple of Christ, the Prince of Peace, is never on the side of those who once wielded the sword and today drop bombs.”

Vance, a former atheist who converted to Catholicism in 2019, questioned how the Bishop of Rome could take such a stance and urged him to “be careful” when opining on theological matters.

“There is more than a thousand-year tradition of just war theory,” the vice president said. “Was God on the side of the Americans who liberated France from the Nazis? Was God on the side of the Americans who liberated Holocaust camps?”

The next day, Johnson became the second GOP leader to cite the longstanding doctrine.

“I was taken a little bit aback just honestly, frankly, by something that was said. I think [the pope] said several days back that something about those who engage in war — you know that Jesus doesn’t hear their prayers or something,” Johnson, a Southern Baptist, told reporters.

“It is a very well settled matter of Christian theology,” he added. “There’s something called the just war doctrine. There’s a time to every purpose under heaven.”

What is just war theory?

Just war theory is a framework of military ethics rooted in ancient civilizations — such as Egypt, Greece, and Rome — that has been refined over millennia by religious figures and scholars.

Though not enshrined in law, it is routinely invoked by Christian leaders and has deeply shaped wartime regulations, from the Geneva Conventions to U.S. military handbooks.

“It is not easy to state what the theory states ‘exactly,’ since it has evolved,” Lisa Sowle Cahill, a theology professor at Boston College, told The Independent. “Its two most influential expressions in the Christian tradition are Augustine and Thomas Aquinas.”

Certain foundational tenets, though, are widely accepted, such as jus ad bellum, which outlines conditions for initiating war. These include: a just cause, adherence to the last resort principle, a high probability of success and authorization by a competent authority.

Once conflict begins, jus in bello — or just conduct in war — governs combatants’ actions, emphasizing distinction between fighters and civilians, proportionality and military necessity.

Does the Iran war pass the test?

Of the four experts who spoke with The Independent, all said that the Iran war fails to meet the criteria laid out by just war theory. The most cut-and-dry violation, they argued, was the absence of a well-defined just reason for initiating the conflict.

“No clear set of causes has been enunciated by the U.S. president and none of the array stated at different times justify the U.S./Israeli unprovoked attack,” Walter Dorn, a professor of Defense Studies at the Royal Military College of Canada, told The Independent.

Since the war began in late February, senior Trump officials have invoked a range of justifications, including that Iran posed an imminent threat to the U.S., a desire to enact regime change and a willingness to follow Israel’s lead.

“Maybe the main argument, at this point, is about preventing nuclear weapons. On my reading of the just war tradition, however, ‘preventive wars’ are not justifiable,” Andrew Fiala, a philosophy professor at Fresno State University, told The Independent.

And claiming that Iran was on the verge of developing a nuclear weapon “requires a lot of evidence before it could be regarded as credible,” Whetham said.

Tulsi Gabbard, the director of national intelligence, told Congress last month that “Iran’s nuclear enrichment program was obliterated” following U.S. strikes last year, appearing to contradict Trump’s earlier claim that Tehran was two weeks away from manufacturing a nuclear bomb. The head of the International Atomic Energy Agency, also said on March 3 that the watchdog group had “no evidence of Iran building a nuclear bomb.”

Other experts contended the war breached the last resort principle — citing previous indirect U.S.-Iran negotiations — and the competent authority requirement, as Congress had not authorized it.

White House spokeswoman Anna Kelly responded: “President Trump initiated Operation Epic Fury to eliminate short- and long-term threats posed by the Iranian regime. Those efforts were overwhelmingly successful – Iran’s ballistic missiles are destroyed, their production facilities are demolished, their navy is sunk, and their proxies are weakened. There is nothing more just than protecting the safety and security of the American people and our troops abroad.”

The Independent has contacted Johnson’s office for comment.

Beyond the war’s origins, U.S. operations in Iran also violate just war theory’s core principles of wartime conduct. Experts specifically cited breaches of proportionality and the distinction between combatants and civilians.

“Massive bombing has killed a disproportionate number of civilians and destroyed civilian infrastructure,” Cahill said.

HRANA, a U.S.-based human rights organization, reported in March that the Iran war has claimed 3,636 lives since it began, including 1,701 civilians. The same month, a preliminary investigation by the Pentagon showed the U.S. was at fault for a deadly strike on an Iranian elementary school, according to The New York Times.

Defense Secretary Pete Hegseth said on March 10 that the U.S. has taken extensive measures to avoid civilian casualties.

Dorn also highlighted Trump’s April 8 Truth Social post, in which he wrote: “A whole civilization will die tonight, never to be brought back again” — along with his prior threats to destroy Iranian bridges and power plants if leaders rejected his demands.

“Trump’s threat to end civilization is one of the most blatant violations of proportionality in the history of modern warfare,” Dorn said.

The Republican president defended his apocalyptic rhetoric by claiming it brought Iran to the negotiating table. Currently, a ceasefire is in place while peace talks continue.

What about past conflicts?

Putting Iran aside, numerous recent conflicts have repeatedly failed to satisfy the principles of just war theory, experts said.

“Contemporary wars rarely live up to the standards,” Fiala said. He did, however, cite the First Gulf War as potentially qualifying, given Iraq’s invasion of Kuwait and the coalition’s success in expelling Iraq’s forces.

“World War 2 is a well-known case. But it was also mixed,” he said. “No doubt the Germans and Japanese were aggressive — so defensive war could be justified. But allied forces bombed cities including firebombing and atomic bombing. Those tactics would seem to violate restrictions known as jus in bello (since the deliberately targeted non-combatants).”

Popular imagination oversimplifies the intricate realities of the war, he added.

“It’s hard for a war to satisfy the criteria — as it should be,” Whetham said. “They are terrible things and should always be an exception.”