It is not isolationist, it is reality. If Russia started backing Mexico and they invaded anywhere along our southern border we would have a MASSIVE logistical advantage over Mexico's backers. The same advantage Russia has over the West in Ukraine.

Russia sees Ukraine as a member of an antagonistic military alliance (NATO) as an existential threat and will not back down from this fight.

We would see Mexico (or Canada**) joining a hostile military alliance exactly the same way as Russia sees Ukraine being defacto NATO (with the constant statements that one day Ukraine will be in NATO). We'd fight to the bitter end to end that possibility.

That is not isolationism. That is Real Politik.

Regarding gas prices I see it slightly differently. Europe WAS paying X for Russian gas, and since about 2008 the US has said this is not acceptable. We blew up Nordstream and now Europe pays Y (which is 3-4 times higher than X) for US LNG. This is decimating European industry and is starting to bring down governments. It is nearing a breaking point.

**Maybe this is what Trump and Obama were joking about at Carter's funeral. Making Canada the 51st state would eliminate the possibility of Canada joining BRICS or a hostile military alliance. /sarcasm