The US is dominated by multiple large industries. Finance and the MIC are two of them.
Blackrock has been heavily invested in Ukraine since at least 2018 (yes, 4 years before Russia invaded). Whatever is left of Ukraine after this war ends (if anything viable) will be in debt to Blackrock for the next couple of generations.
US MIC stocks went up massively thanks to Russia's invasion.
Those two groups (Finance and MIC) are two of the biggest donors to US politicians and political parties. They are also two of the biggest sponsors of US MSM (pharma is biggest, IIRC).
There is your motivation. Follow the money.
Boris Johnson called Zelenskyy and then flew to Kyiv in March 2022 specifically to tell Zelenskyy that the US/UK would NOT support a peace plan.
December 2021 Biden refused to even speak to Putin about peace.
2015 to 2022 Ukraine refused to implement Minsk II - which is against international law since Minsk II was ratified by the UN.
Poroshenko, Hollande, and Merkel all admitted that they never intended to implement Minsk II. It was all a delaying tactic to rearm Ukraine in preparation for this war. The US MIC was the main supplier and benefactor. The US sent over $3 billion in weapons to Ukraine from 2015-2022.
The US and the USSR almost went to war in 1962 because of missiles in Cuba. The weapons and missiles of 2023 are orders of magnitude more powerful than in 1962.
Let's say Mexico wanted to join the Russian Federation. Russia has troops on the ground for 8 years training and arming Mexicans. They hold mock battles where every time the "enemy" is the US, and they train how to counteract US weapons and tactics. During all of this, Mexico is outlawing English, closing down any American-leaning media, outlawing American leaning political parties, and killing American leaning Mexicans - by the thousands. To say that the US would not say that is a security threat is ridiculous on it's face. That is actually less bad than the situation in Ukraine since 2014.
The war is sadly long from over. What the US (meaning NATO) will do is unknown. Right now Russia is winning. Definitely not without losses, but winning all the same.
Ukrainians elected Zelenskyy in 2018 specifically because he said he would achieve peace with the Donbass and end the civil war. Of course there are lots of Ukrainians willing to fight and die to expel the Russians now.
I have no idea what Putin thinks. I think that unless some other major force enters the war on the side of the Ukrainians, we're looking at months to years of grinding war moving west across Ukraine, killing 10s to 100s of thousands and displacing more millions than those who have already fled.