If the president wants to launch a nuke, the only thing that stops them is their cabinet simply not relaying the order. This happened several times when Nixon got shitfaced.
Like nearly everything in american government, this is just a "norm". The only thing preventing an insane american president from doing pretty much anything, at least for a while, is "norms" and customs. Nixon wanted to nuke pretty much all of Vietnam pretty much all the time, and it was only his cabinet that prevented that. But there is no precedent, or legal reason, to actually force that to be the case. So what happens if you then get a president who has surrounded themselves with a cabinet that explicitly thinks Nixon was right, that Nixon had the right to burgle the Watergate hotel because the president should be able to do anything (this is a real faction in US politics right now) and have openly put together a plan to fire damn near everyone on day one and replace them with explicit yes men?
As for Nixon, he was literally playing the madman as part of a strategic goal [2] (arguably the exact same thing North Korea is doing in modern times), but privately was the one who ultimately scrapped a plan that involved nuking Vietnam. [3] A nice quote from the Madman article (from Nixon), "I call it the Madman Theory, Bob. I want the North Vietnamese to believe I've reached the point where I might do anything to stop the war. We'll just slip the word to them that, "for God's sake, you know Nixon is obsessed about communism. We can't restrain him when he's angry—and he has his hand on the nuclear button" and Ho Chi Minh himself will be in Paris in two days begging for peace."
[1] - https://www.dailymail.co.uk/news/article-5082689/Retired-gen...
The Project 2025 agenda targets the Pentagon as being "woke" and full of "Marxists" and will likely result in a purge of generals and officers. Anyone who has ever signed off on anything they can call a diversity program will get ousted on day 1. The vacancies will then be filled with Trump supporters.
This makes one of the biggest threats to humanity's survival being buggy software. If a software glitch from one side or the other suggests nuclear weapons are incoming, they will almost certainly retaliate in kind. And once the bombs are away, the other side will definitely respond, and that's pretty much gg humanity. In fact this exact thing very nearly happened in 1983. Soviet instrumentation indicated that the US had attack the USSR with multiple nuclear weapons. The technician on duty, Stanislav Petrov, [correctly] judged that what was happening was an instrumentation error, and disobeyed protocol - refusing to escalate it to his superiors who very well could have ordered retaliatory strikes.
For some context this had happened at once of the highest levels of tensions between the US and the USSR. This was in the era of the 'Star Wars program', which was to be a US missile defense shield, leading the Soviet Union to become paranoid about intentions of a preemptive nuclear strike attack. Those tensions had already led to the downing of one 747 that had inadvertently veered into Soviet territory, while carrying a US Congressman at that. Given this context it's highly probable that his superiors would have ordered the retaliation. So he's one of very few people who can be reasonably said to have saved the world.
[1] - https://en.wikipedia.org/wiki/Stanislav_Petrov