In April, 2017, President Trump was enraged after the Syrian President, Bashar al-Assad, used chemical weapons, outlawed by international treaty, against his own people, including children. Crude videos transmitted on cell phones from the town of Khan Sheikhoun, showing the injured and dying—as they gasped for breath, writhed in spasms, or foamed at the mouth—were gruesome. The United States responded by firing Tomahawk missiles at the airbase used by the Syrian warplanes that dropped the toxic bombs. But the President wanted to do more. “Let’s fucking kill him!” Trump said, according to Bob Woodward’s dramatic account in his new book, “Fear: Trump in the White House.” “Let’s go in,” Trump urged. “Let’s kill the fucking lot of them.”

Trump has angrily denounced the book as “fiction”—and, in a tweet, as the “exact opposite of the fact.” Yet the tone of Trump’s purported comments in Woodward’s book echoes his confrontational tweets about world leaders and U.S. foreign policy. With foes, he’s tweeted threats of war. In July, he wrote that Iran faced “CONSEQUENCES THE LIKES OF WHICH FEW THROUGHOUT HISTORY HAVE EVER SUFFERED BEFORE.” Before North Korea’s outreach in January, Trump tweeted an apparent order to his Secretary of State that diplomacy with the “Little Rocket Man,” Kim Jong Un, over his nuclear-weapons program, was a waste of time. “We’ll do what has to be done.”

As distasteful and abnormal as such tweets may be, they are within the powers of the Presidency. But assassinating a foreign leader is not—unless Trump changes the rules enacted or embraced by his seven predecessors. In 1976, President Gerald Ford issued the first executive order on the matter, which stipulated, “No employee of the United States Government shall engage in, or conspire to engage in, political assassination.” Presidents Carter and Reagan toughened the terms in their own executive orders.

Using surrogates is illegal, too. “The United States can’t authorize or encourage or support an assassination that it couldn’t carry out itself,” John Rizzo, a long-serving lawyer for the C.I.A.’s Directorate of Operations, the Agency’s clandestine branch, who has twice served as the acting general counsel for the C.I.A., told me. “You can’t nudge a foreign opposition group—for example, during a coup attempt—to assassinate a foreign leader. You can’t outsource an assassination.”

The loophole—which Trump could exploit to change U.S. policy—is that Congress has never passed a law on political assassinations. “Since 1974, it’s been understood that assassination is off the table. I don’t know of anyone who says, ‘Why don’t we kill that person?’ It just doesn’t happen,” John McLaughlin, who served more than thirty years in the Directorate of Intelligence and rose to become the acting director of the C.I.A., told me. “But an executive order is just an executive order—and this one is not embedded in law.”

Technically, all Trump would have to do is issue a new executive order to supersede the earlier ones. “It’s not an executive order you’d change lightly,” McLaughlin added. “You’d have a lot in Congress—or at least some—who would say that’s breaking a long-held ban.”

The prohibition on assassinations emerged out of the ugly history of U.S. plots against foreign leaders—waged directly or through local opposition groups—during the early years of the Cold War. Some schemes were bizarre, others half-baked. Some even evolved out of vague interpretations of a President’s priorities, without explicit or written orders from the White House.

On August 18, 1960, President Dwight Eisenhower told a meeting of his National Security Council that he was deeply concerned about Patrice Lumumba, a popular and charismatic former postal clerk who had been inspired by the writings of Rousseau and Voltaire to lead an independence movement in the Congo, a Belgian colony in the strategic belly of Africa. Lumumba, who became the first Congolese Prime Minister, had visited Washington, but he also threatened to turn to the Soviet Union when Belgium’s military forces were slow to leave his country.

The C.I.A. chief, Allen Dulles, interpreted Eisenhower’s alarm as an order to assassinate Lumumba. Dulles cabled the C.I.A. station chief in Leopoldville, now Kinshasa, that Lumumba’s death was “an urgent and prime objective.” A top C.I.A. scientist was dispatched with a biological weapon made at Fort Detrick, Maryland—along with hypodermic needles, rubber gloves, and gauze masks—that “would produce a disease indigenous to Africa,” according to a 1976 report by the Senate’s Church Committee, which exposed decades of C.I.A. abuses. The goal was to somehow get Lumumba to ingest the material, in his food or even in toothpaste. A local C.I.A. agent later testified that he had suggested simply shooting Lumumba, but the U.S. wanted plausible deniability—and a killing without a trace of its involvement. It failed. Lumumba was murdered by Congolese political rivals, who put him in front of a firing squad.

Between 1960 and 1965, the United States repeatedly plotted to kill Cuba’s Fidel Castro. One early attempt involved poisoning a box of Castro’s favorite cigars with a botulinum toxin. Another involved hiring underworld figures, including the Chicago mob boss Sam Giancana and a Cuban associate, both of whom were on the F.B.I.’s “Ten Most Wanted” list. The C.I.A. provided a half dozen poisonous pills to get Castro to ingest, but an intermediary backed out of the plan.

The U.S. facilitated, had an indirect hand in, or was implicated in other assassination plots, including the murder of Rafael Trujillo, the dictator of the Dominican Republic, in 1961. The U.S. provided dissidents with pistols, carbines, and other material support, although it turned down a request for machine guns. The United States also supported a coup against Ngo Dinh Diem, in South Vietnam, in 1963, that ended in his death. In 1970, the U.S. provided machine guns, financial aid, and other equipment to Chilean military officials who kidnapped—and ended up killing—General René Schneider, the Army commander.

“The evidence establishes that the United States was implicated in several assassination attempts,” the Church Committee investigation concluded. “Once methods of coercion and violence are chosen, the probability of loss of life is always present. There is, however, a significant difference between a coldblooded, targeted, intentional killing of an individual foreign leader and other forms of intervening in the affairs of foreign nations.”

The 1976 Senate probe led the Ford Administration to issue the first ban. The order has stuck. The one exception is if a foreign leader presents an immediate national-security threat to the U.S. during wartime. “The Clinton, Bush, and Obama Administrations all concluded that the assassination ban in Executive Order 12333”—issued by President Reagan—“did not prohibit targeting or killing a foreign leader, or indeed any person, provided the action was taken in self-defense or in the course of an armed conflict,” John Bellinger, who served as the legal adviser at the National Security Council and the State Department during the Bush Administration, told me. “In other words, the United States could target a foreign person if that person was either engaged in an armed conflict with the United States or preparing to launch an attack on the United States.”

It’s a very precise legal interpretation, however. For example, Russia’s intervention in the U.S. election may have challenged U.S. national security, but Rizzo told me that “the United States couldn’t kill Vladimir Putin, because he’s a leader and there’s no state of war.” The same applies to the Syrian leader. “Unless the executive order were rescinded by the President,” Bellinger said, “a targeted attack against President Assad in response to Syria’s use of chemical weapons would have violated these prior interpretations of the assassination ban, because the United States was not engaged in an armed conflict with Syria, and Assad was not planning attacks against the United States.”

So, for now, Trump has no legal authority to target Assad. But he does hold the power to change the rules, principles, and foreign-policy doctrine that date back more than four decades.