The Russians aren't going to just leave on their own, which is why Ukraine's regular military forces are attacking them constantly in one of the more active wars going on in the planet. In response to that, at some point the Russians are likely to come to the determination that they will need to withdraw.

This is the assumption the left wing position is based on.

It's based on history. Let's paint your scenario. Ukraine gives up what territory is necessary to gain peace. Trump decides he'll sell arms to
Europe, but not provide anything more to the Ukraine. Russia consolidates, then takes Moldova. Then protrudes a psuedopod to the Baltics - which is RIGHTFULLY RUSSIAN. It's constant little wars, little green men in places, and loads of propaganda about how the Euros are being aggressive by not letting Putin have what he wants. With each loss, the dream of reconstituting the USSR gains fresh support. We need to have the Russians put that dream behind them and concentrate on developing the north - to their economic advantage. Why? Because we are conducting the great experiment to find out what the earth is like after we change the climate.