Back when Trump ran things, he was pretty much a carry a big stick type of negotiator. He didn't have a history of foreign policy moves that other world leaders could look to in order to gain some sort of yardstick of how he might react. But he talked tough. And the enemies of the US took notice. He went to North Korea. I have to imagine that he didn't use a lot of flowery words or speak of abstract concepts of rainbows and unicorns. It was probably **** around and find out rocket man.
Same with Russia, Iran, China, the palestinians. Don't start no crap and there won't be no crap. When you have a potent military, those threats are easily backed up. A strong military makes for peaceful neighbors. Even the far away neighbors.
Then the US political climate changes as far as leadership. You have the Afghan withdrawal that the entire world witnessed. No matter how they tried to spin that, it was a colossal failure. And it enriched our enemy with the latest gadgets of modern warfare fighting technology. So much for a person with a 40 year history of politics.
Russia took notice of biden's impotence. Plus on the heels of that disaster, the US appetite for all things military overseas was at rock bottom. No way were we going to get involved after a 20 year quagmire was put out of it's misery. I believe the only way we would have gotten involved in any sort of direct military conflict is if we had been attacked directly. Otherwise America was going to sit out the next war. I think that is what the Russians perceived would be the reaction from Washington. I'm sure when Russian troops were massing on the border there were some back channel negotiations going on that the American citizens are not privy to.
But in the end Russia took a chance. If I can believe the news reports of the war over there, the reds may have underestimated their foe. But that happens. It appears the Ukes aren't like the French. They will actually take up arms to defend their homeland. And they may lose, but not without a fight and taking a few tens of thousand of Russians with them to Valhalla.
TL/DR version. If Trump was still in the oval office, Russia plays nice with it's neighbors and doesn't attack.
Also if you believe what you read, Russia has already lost more soldiers in a few months than the US lost in the entire dozen of years we were involved in Vietnam. 93+K killed to ~58K US military killed.