It's not that he threatens things he can't go through with. It's that he presents himself as unhinged and capable of any sort of messed up thing. So when he s

It's not that he threatens things he can't go through with. It's that he presents himself as unhinged and capable of any sort of messed up thing. So when he says he's going to send troops to Panama to retake the canal, he probably doesn't expect he'd ever actually do it. But Panama's government can't be sure, so when Trump asks for something perhaps a bit more "reasonable" than returning the canal to US control, Panama will look at that as better than risking a military incursion, and capitulate. This has already happened with lower-stakes stuff, like the tariffs on Canadian and Mexican goods that Trump never even put into effect once he announced them but immediately got concessions from Canada and Mexico. Whether or not those concessions are actually useful or will materialize is another matter, but that's almost irrelevant: Trump knows he can push them around now, and pretty easily at that.