If it weren’t for the existence of Russia’s massive fleet of nuclear weapons (which probably don’t actually work but it’s a hell of a risk) i’m sure NATO would have already got involved.

The only reasonable way I can see this war ending is if the internal politics of Russia decide Putin is now too much of a problem and deal with him themselves. Then whoever takes charge backs out of Ukraine because it’s not a winnable conflict.

I can’t see how anyone external can bring about an end to the conflict without risking massive escalation.