Jerry Pournelle has some experience thinking about incidents like the Benghazi Affair:
As to what might have been done: we knew that conditions in Benghazi were deteriorating long before September 11, 2012, and also that 9-11 is a memorable anniversary. Benghazi is a seaport. The United States has ships called helicopter carriers. Helicopters land on them. Helicopters can carry Marines. Marines carry rifles in addition to bayonets and ammunition. Marines have been known to defend US diplomatic institutions against local attack, and their effectiveness is well known.
As to obtaining Libyan permission to allow a US helicopter carrier — say something like the late USS Tripoli — to berth in Benghazi, their refusal would itself be something worth knowing, and they certainly have no capability of preventing such a ship to stand off shore; and surely the Libyan provisional government would not refuse a US request to send in helicopters to rescue American diplomatic personnel. We could negotiate that in advance of 9-11, and doing that would be simple prudence.
It is not as if we have not gone through such things before. The United States has some experience in these matters. And we did have warning in advance.
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Back in the 1950’s I partnered with some senior officers of the US Navy to write papers on international security. Our conclusion, based in part on experiences of the British Royal Marines in Africa, was that ships capable of carrying about a battalion of Marines with helicopter transportation and air support would be capable of handling the vast majority of violent incidents taking place within 100 km. of deep water. Those papers are said to have been influential in the design and commissioning of the Iwo Jima class helicopter assault carriers including the USS Tripoli which served as President George H W Bush’s flagship when he visited Somalia. My son Phillip was an ensign on the Tripoli at that time. I don’t claim much credit in ship design, but I did have some small input in development of tactical use; count me as a fan of Iwo Jima class as a means of force projection. Alas, they are pretty well gone, and I am not all that impressed with what replaced them.
I still believe that the Navy needs the modern equivalent of the Iwo Jima class ships: a battalion of marines with transport and fire support. Their purpose is to deal with situation like the Benghazi incident — largely by deterring them. The threat of an avenging company of Marines not an hour away is a very effective deterrent to terrorists planning an armed attack. They can’t stop a suicide bomber, but they can sure stop a group with AK-47’s, RPG’s, LMG’s, and mortars.