Getting a force put together capable of doing credible work on real unconventional warfare missions is pretty difficult, and way outside the capabilities of a MEU(SOC) or anything the USMC had on the books during the Cold War. It's definitely not a mission set where you can mass produce guys on a fast timeline, so those sorts of guys would probably have to be part of some fictional unit the USMC stood up prior to the war.
Though having the embassy guards doing double duty as Jedburghs would still leave the issue of who would provide security for the embassy. Would probably be easier just to bring in SF guys to work out of the embassy -- they've certainly got a lot of experience doing such in whatever place Congress said we couldn't have troops but we needed to prop up an ally over the last 30+ years.
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