The RAND Corporation published an interesting little report last week that has quite a bit of relevance to the physical security of U.S. diplomatic missions. Read it here: Seizures of Western Diplomatic Facilities, Historical Timelines, 1979–2019.
It addresses these research questions:
What were the durations of attacks on Western diplomatic facilities since 1979, and how much advance warning was there of each attack? And,
What implications do historical timelines of duration and advance warning of attacks on diplomatic facilities have for efforts to respond to such attacks?
The report found there have been "33 successful seizures of Western diplomatic facilities since 1979 ... The majority of attacks culminated in two hours or less, and over 90 percent culminated in six hours or less ... the median attack duration was four hours, and the average was 4.8 hours."
Mind you, those incidents were not only at U.S. diplomatic missions. My general sense of the history of these things is that you'd see a longer average duration if only U.S. missions were considered.
RAND supposes that "the lengthening of this [attack] duration could offer wider windows of opportunity to intervene," such as, intervention by U.S. military forces. Hum. Does that sound likely?
RAND is the Defense Department's think tank, so far be it from me to question whatever they say about the chances of U.S. military intervention to an attack underway at a U.S. diplomatic mission. But, I can legitimately point out that the last time there was such an intervention was in 1900 during the Boxer Rebellion in China. Can you think of another? I can't.
There was an attempted intervention in Iran in 1979 - operation Eagle Claw - but it failed. There has not been another one since, and for what should be an obvious reason. The U.S. military is not a police force, and a U.S. embassy cannot just call 911 and expect an immediate response from the SWAT team.
The closest thing to a military SWAT team is the Crisis Response Force (formerly known as the Commander's In-Extremis Force) and those, as we learned from the Benghazi Accountability Review Board report, are prepared to respond to a crisis within six hours. On the night of the Benghazi attack, the closest In-Extremis Force was training in Zagreb, Croatia, and the attack was long over before they could have arrived.
The realities of time and space make it completely unrealistic to expect a timely intervention from the U.S. military to an attack underway at any of the 270+ U.S. diplomatic missions around the world. The Select Committee on Benghazi heard testimony from both the Secretary of Defense and the Chairman of the Joint Chiefs of Staff to that effect. Does anyone doubt it? If so, then I think you read too much Tom Clancy and play too many video games.
All that said, please do read about The China Relief Expedition Joint Coalition Warfare in China Summer 1900, which is even better than the Charlton Heston movie version.