Mexico is a frequent counterexample, but I'm not sure what would happen if, say, USSR would still exist, and would manage to create a union with Southern Americ

Mexico is a frequent counterexample, but I'm not sure what would happen if, say, USSR would still exist, and would manage to create a union with Southern American and Central American countries. Like, Cuba, then Venezuela, then, imagine, Colombia... I wouldn't think that USA would immediately jump to nukes.