Since John F. Kennedy, every president has had an officer that follows him around with the so-called “nuclear football,” a briefcase that can be used to launch a nuclear attack (it got its nickname from a nuclear war plan called “dropkick”). This is something the president would do not with a button but with his personal nuclear codes, which he also must carry on him at all times.

It’s a pretty big decision to place in the hands of one person, and an executive power that Congress has challenged under President Donald Trump’s administration. So far, no president has ever actually used the football—but still, why does the decision about starting nuclear war come down to the discretion of just one person?

Interestingly, the only president in history to approve a nuclear attack—Harry S. Truman—wasn’t actually very involved in the decision. Although he knew an attack was planned, military officials executed it on their own. Truman was on a ship when the first bomb dropped on Hiroshima on August 6, 1945. He didn’t hear about the actual bombing until roughly 16 hours later, after he’d already spent some time relaxing on deck while a band played.

Alex Wellerstein, a professor of science and technology studies at Stevens Institute of Technology, says Truman might not have known about the August 9 bombing of Nagasaki in advance. “I don’t think there’s a lot of evidence that he realized that they had two bombs ready to use so quickly,” says Wellerstein, who runs a blog about nuclear security. “He certainly wasn’t given any heads up about the second attack.”

Yet that soon changed. The day after the Nagasaki bombing, the military told Truman that they could have another bomb ready within a week. Faced with a possible third bombing, Truman immediately asserted control over the situation, declaring that more bombs could not be used without presidential approval. He also curtailed the military’s access to these new and frightening weapons.

The next president, Dwight D. Eisenhower, started moving things in the other direction by expanding the military’s access to nuclear weapons. But right after, President Kennedy once again reduced this access. It was something his administration had started to do before the Cuban Missile Crisis, but became much more concerned about afterwards.

“One of the things they take away from the crisis is … what if one of these young airman had thought he saw Cubans coming over the horizon and started attacking?” Wellerstein says. “You could’ve had nuclear war by accident, which sounds even more terrible than nuclear war on purpose.” There was also the fear that some rogue official could start a nuclear war all by themselves—a concept explored in the 1964 film Dr. Strangelove.

By the end of his administration, Kennedy was being tailed by a man carrying an early version of the nuclear football that contained a list of phone numbers to call and a series of attack plans for him to pick. It’s not clear when the public found out about this, but as early as 1965, The Baltimore Sun was calling it a “football” with nuclear capabilities. That same article described how the man who carried the football for Kennedy even followed him to the hospital after the president was shot.

Throughout the Cold War, presidents carried the football with them in case the Soviet Union launched a surprise attack. Because the U.S. would only have minutes to respond, it seemed reasonable to have the president travel around with it. Wellerstein says that Nixon’s excessive drinking and increasingly erratic behavior at the end of his term is one instance in which an administration questioned its commander-in-chief’s ability to handle the football. Yet the concern around Trump is, quite simply, unprecedented.

In February 2017, many were disturbed when a guest at Trump’s Mar-a-Lago resort posted a picture of himself on Facebook standing next to the officer who was carrying the briefcase that allows the president to launch nuclear weapons at any time. Yet experts said this was not as dangerous as the fact that, on the same weekend, Trump held a dinnertime meeting about North Korea’s nuclear threat on the resort’s open-air terrace.

The idea that the president had to approve nuclear attacks, says Wellerstein, was never actually put into law. Over time, presidential directives established a protocol for launching nuclear weapons that generally assumed the president had sole authority to launch them. During Trump’s first year, one prominent Republican speculated about whether Trump’s team would ever tackle him to prevent him from using the football.

But generally, it is assumed that if the president uses his codes to authorize a nuclear strike, it will go through unquestioned. That is, after all, how the system was designed to work in the first place.