Naked Capitalism has a long article which links to a lot of other long articles.

SOCOM carries out the United States’ most specialised and secret missions. These include assassinations, counterterrorist raids, long-range reconnaissance, intelligence analysis, foreign troop training, and weapons of mass destruction counter-proliferation operations[.]

One of [SOCOM’s] key components is the Joint Special Operations Command, or JSOC, a clandestine sub-command whose primary mission is tracking and killing suspected terrorists. 

Naked Capitalism

Training other countries’ militaries at their invitation seems okay. The question is always who is running that country, what is their human rights record, etc. Tracking and killing terrorists is problematic because just designating someone a terrorist makes it okay to kill them. Like for example, any individual involved with an “Iran-backed group” in Syria, Yemen, or wherever. Which are countries our troops are most definitely not invited to be in. If you have troops on the ground in a sovereign country that you are not invited to be in, how is that not a war against that country? These are often “disputed territories” of one sort or another, but again if you can just label as place you want to invade a disputed territory and then invade it (as Russia has done repeatedly), that is clearly problematic.

Finally the really scary thing about special forces is they can kill politicians and take over countries, if they want to. Including ours. Let’s hope they don’t want to, but if someone ever wants to do that and has control over the special forces, this is the direction a straight-up military coup could come from. They would just come up with a narrative that it is necessary to “protect the constitution”, and the demands of the dead founding father-gods would then trump whatever living civilians are actually nominally in charge at that moment. Don’t worry about the other branches of government – the Cowardly Congress will go along with it, and the Supreme Court will decline to hear the case.