So when trump says Ukraine has to make concessions to achieve peace... The first thing that comes to mind is that the allies did the same thing when the Nazis started invading their neighbors in the 1930s. Peace at any cost. It showed that all they had to do was threaten war to get what they wanted. In the end, the cost to stop them was WW2.

My point is that the aggressor must pay a steep cost and still not get what they want.