(For reference, I was heavily inspired by the latest episode of the Korean adaptation of Designated Survivor, Designated Survivor: 60 Days)
Let’s say that, for whatever reason (nuclear attack, catastrophic terrorist attack, extreme natural disasters, Godzilla going apeshit on Ghidorah, etc.), a friendly, democratic nation that’s a strong and very solid as well as well-trusted ally of ours and who’s hosting a sizable American military presence (let’s say that this nation is hosting around 15k soldiers, sailors, Marines, and airmen) suddenly experiences a military coup led by the top flag officers of their military. Their capital city is occupied by their military and their now military-led government imposes nation-wide martial law effective immediately. The US has no involvement in and/or knowledge of the coup and no one saw it coming.
In such a situation like that, what would the US military forces stationed in the country do in response to this coup? What about our diplomatic presence there? Is the US Embassy in the capital city put on lock down? Are US troops recalled back to base and would the base be locked down? How would Washington react upon hearing that a strong, democratic ally of ours just experienced a military coup? What would/should American citizens do in that scenario?