The rest of NATO continues to support Ukraine, in spite of Trump ALREADY blocking the US . So he can't just give Ukraine to Putin. But it makes it a lot harder for them and it means the US isn't sharing in the cost as it has been, although not disproportionately.
Obviously, this suggests the end of US leadership in defending freedom. It already means it because the GOP is able to obstruct your participation. And even though Biden is willing to help, he's mostly following other countries (tanks, missile, fighter jets), not leading them.