I'm sorry. Let me throw out the hypotheticals in my head and these are not based on any real situation.
1. What if a small Eastern European country created a large nuclear reactor but didn't care about safety meaning that the probability of a meltdown that would effect Europe was X percentage. At what value of X is it acceptable to invade assuming all other methods of persuasion have been attempted.
2. If a country is polluting Y amounts of something that increases global warming by X and all methods of persuasion have stopped is invasion ever justified at some values of X and Y. You are considering the probability of and level of suffering of two events.
A. The invasion
B. The suffering caused by environmental damage.
There has to be some conditions in which it is morally justified to invade a country
Considering Russia did #1 in Chernobyl and all they got was their own (well Ukraine’s) land irradiated and a lot of strongly worded letters, the bar is practically a lot higher than you seem to think it should be.
In a dictatorship the population is responsible for not overthrowing the government.