Ukraine

It's somewhat analogous to the policy the US used with Finland around World War 2.

In the Winter War, the Soviet Union invaded Finland, attempted to annex it. The US provided financial support and some other aid to Finland. Then Germany attacked the Soviet Union and the US wound up on the same side of the war as the Soviet Union. They were both on the Allied side. However...the Soviet Union and the US didn't agree on Finland.

Stalin demanded that the UK and US attack Finland. The UK did some rather limited pro forma airstrikes. The US told Stalin that he could shove right off with that.

As long as Finland was defending its own territory, the US was okay with it. But then Finland -- who was conducting an offensive in conjunction with Germany -- pushed back Soviet forces far enough that it could head outside its borders into the Soviet Union, and even more problematically started heading towards cutting supply lines linking supplies from the US to Soviet troops fighting against invading German forces.

That was a different story.

The US told Mannerheim that if Finland started cutting into the Soviet Union and severed those supply lines, that'd be it -- it would enter the conflict against Finland. It'd support Finland defending Finnish territory, but not hauling off into Soviet territory.

Is a policy of limited support in Ukraine the right one? I don't know. But I don't think that limiting support, saying that one can fight forces that have invaded without restriction but can't go clobber things in the other country with said weapons is intrinsically unreasonable. The "you can use weapons that cross the border to hit Russian forces that are taking advantage of that policy to attack you from safety in Russian territory" isn't really a fundamental rewriting of that policy. It's just refining it so that Russia can't exploit the border to stage forces.

I don't think that it is likely that the US will move on this policy. That is, the US has probably decided, after a lot of deliberation in the bureaucracy among experts, that it is not looking to have a war in Russia. As long as Russia is sending forces into Ukraine, then those are fair game. If Ukraine wants to cut into Russia with its resources, okay, the US doesn't own those resources. If someone else wants to provide resources to do that, okay, that's their resources.

I think that Ukraine has decided that -- especially as Russia is attacking their power generation infrastruture -- that conducting a strategic campaign against power generation infrastructure and fossil fuel infrastructure in Russia is fair game. Okay, that's their call. But I don't think that that's something that the US is bound to back; the US can say "we don't want to have a strategic bombing campaign against Russia's infrastructure". The flip side of "the US doesn't tell Ukraine what to do" is "the US isn't obligated to back every policy that Ukraine adopts".