One day late last winter, I went to Andrews Air Force Base outside Washington and boarded the Doomsday Plane. The plane is a hardened 747, and its real name is the National Airborne Operations Center; its core mission is to be the unbreakable communications link for the president in the case of a nuclear attack on our country.

I learned a lot that day as we flew west across America – how the systems functioned, the redundancy of the technology and, especially, the skill and dedication of the young military personnel who made it all work. But it wasn’t until a simulated attack burst onto the screens in front of us that the real lesson of this trip sank in.

The decision to launch nuclear weapons in response to attack or risk losing them to incoming missiles could well come down to minutes – not hours – and even though I was observing an exercise, I was struck by how quickly the clock counted down.

As we worked through the simulation, I realized the kinds of questions the president would face in the event of this kind of crisis. Is it really an attack? (Early in the Cold War, legend has it, a flock of geese flying near the North Pole activated our early warning systems.) Is the target the U.S. or one of our allies? Where, specifically, would the bombs or missiles hit? What kind of weapons have been fired?

Who is behind the attack? Could we establish contact with the government of the attacker? Where are our weapons and which ones are ready to fire? What should our targets be? How large should the counterattack be? How much time do we really have?

And what hit me with almost physical force was that during the first few minutes of such a crisis, all these facts and critical questions would have to be sifted, complex data analyzed and the most important decision in world history made – in a matter of minutes – by one lone person, who could well be sitting in an evacuation helicopter.

One lone person.

Most of us think of our government as a complicated system of checks and balances, a frustrating collection of voices, opinions and agonizingly slow decision-making. Committees, agencies, courts, elections – every decision is subject to input, debate, examination, vetoes, appeals and amendment. Every decision, that is, except one.

In the most important decision of all, literally the fate of civilization, there is no national debate, no poll, no checks and balances, no election, no Congress, no Supreme Court, no Jack Bauer. Just the president, under duress, getting an avalanche of conflicting and confusing data and advice from all over the world, as the clock ticks down.

And nothing that I have seen over the 18 months of this presidential campaign – or in the 30 years of his semi-public life – has convinced me that Donald Trump should be the person, the lone person, making that fateful decision. In fact, what I have seen is glaring evidence that the opposite is true, that he would be the last person you would want in that position. At a moment like this, walls, emails, tax returns or shaking things up look a lot less important, and it all comes down to temperament and judgment and the ability to process a huge amount of information in a hurry to reach a decision unclouded by emotion.

Over and over in this campaign we have seen the opposite from Mr. Trump – late-night tweets reacting to every taunt or slight; intemperate rants about the crooked media, the incompetent generals, the rigged election or a shadowy international conspiracy; thinly veiled incitements to violence, or a breathtaking recklessness about everything from torture, to our alliances, to the spread of nuclear weapons.

I know that Hillary Clinton is not a perfect candidate; there is no perfect candidate on the ballot this year (and there never will be), but she is smart, focused, tough-minded and knows the traps and pitfalls of our dangerous world.

She has made and participated in hard decisions at the highest level. One CIA official who watched her in the White House Situation Room under intense pressure reported that she was “prepared, detail-oriented, thoughtful, inquisitive and willing to change her mind if presented with a compelling argument.”

That’s the person I want in that helicopter, not Donald Trump.