For the first time since the Cold War, Americans are worried about their president’s ability to handle the nuclear arsenal. Just two days into 2018, President Donald Trump bragged on Twitter that his “Nuclear Button” was “much bigger & more powerful” than North Korean leader Kim Jong Un’s. For some, this was a troubling reminder of Democratic presidential candidate Hillary Clinton’s warningthat “a man you can bait with a tweet is not a man we can trust with nuclear weapons.”

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 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.