In 1903, Panama separated from Colombia. Colombia tried to forcefully retake Panama, but Panama quickly allied with the U.S. which easily prevented the reunification. Some view this as the U.S. essentially taking over that region.
Nowadays, directly taking over a region would be considered a war of aggression, which would cause U.N. members to implement sanctions against the aggressor (in theory). My question is, does international law say anything about recognizing secession governments?
As an extreme example, a government could just ask someone in some region to declare independence in exchange for military protection of their new government and an alliance. Here is a more concrete scenario. I am Pennsylvania. If I formed a secession Pennsylvanian government and entered into a military alliance with China, would that be a breach in international law? Obviously the U.S. would try to destroy my government, not even attributing any statehood to the government I formed. Would third parties be required to sanction China, however? If my new country was successful (with the help of China), would it also get sanctioned (let's say my uncle in New Jersey also forms a secession New Jersey government and agrees to give my new country free transport of goods through the east coast)?
If not, what are the limits of recognizing secession governments? I can of course make the above scenario more reasonable if instead of China and Pennsylvania, we use New Mexico and Mexico, or parts of FSU states and Russia. Or, on the topic of U.S. states, consider the scenario where the current Pennslyvanian government allies with China.