There are plenty of legitimate governments - and to be clear, by “legitimate” we usually mean the government recognized by the international community, whether or not any given people think they’re good guys or whatever - who do not control all of the territory they claim.

The point is that if a territory is under control of a foreign or rebel group and is attacking international civilian or military assets, then the international community can respond if the country that has claims to the territory cannot. I’m not even sure that the Yemeni government is in a position to coordinate strikes at this point, but that would be the standard approach otherwise.

If the Proud Boys took over south Texas and started launching military attacks against Mexican military facilities, and the US government was unable to stop them, Mexico and the international community would be within their legal rights to stop them.