If dictator, fascist loving Trump becomes President ever again, what's to prevent him from allying with Russia? A lot. The democratically aligned NATO nations would protest. But, what would prevent Trump from slow l-walking a move toward a break from our current national security allies? Not much.

If Trump were to become POTUS and to have mostly the kind of Congress he wants, he could effectively dismantle America's National Security structure from top to bottom.

He'd likely start with stopping all funds and support for Ukraine. Then, he could call for other countries to do the same.

Further, to help Putin undermine Western democracies, he could ally with the Russia-leaning leaders of Hungary, Slovakia, Serbia, and Austria, and attempt to rally support for Putin-friendly opposition candidates in Poland, Germany and France. He could also take security clearances away from former national security leaders and military officers who disagree with him and stack the Pentagon with fascist & Russia friendly officers and career civilians.

He could also close important bases and decommission important military units and continue to "accidentally" (on purpose) disclose sources and methods of US & allied intelligence. What happened when he did that his first time in office? Nothing. He also kicked out everyone, including his translator, to have a private 20 minute conversation in private with Putin. (It's doubtful Trump can speak any foreign language except maybe German, but Putin is fluent in several languages, including English and including the language of manioulation, especially in manipulating self-serving, low IQ narcissists.) Clearly, under Trump,
national security secrets are at risk.

If Trump fills government and military with his acolytes if he becomes POTUS, the damage he could do to our allies' and our own national security could be immense and possibly permanent.