On October 16th the Central Intelligence Agency (CIA) delivered this urgent memo to the State Department and the White House, revealing that U-2 flights had photographed evidence of Soviet medium range ballistic missiles in Cuba:

Memorandum: Probable Soviet MRBM sites in Cuba

“Photography of October 14th 1962 has disclosed two areas in the Sierra del Rosario mountains, about 50 nautical miles west-southwest of Havana, which appear to contain Soviet MRBM [medium range ballistic missiles] in the early stages of deployment. A third area, about five and ten miles east of the first two respectively, appears to be a military encampment. The first site includes 14 large tents, 15 smaller tents and 75 vehicles of a number of different types…

The second site is five nautical miles east of the first and… contains four specially configured vehicles or pieces of equipment which could be used for missile erection in a field environment. At the time of photography, one of the trailers was in juxtaposition with one of these possible erectors. This site also contains 17 large tents, 20 small tents, 10 large trucks, 16 small trucks and 12 unidentified pieces of large equipment…

The dimensions of the trailers indicate that either the SS-3 or SS-4 ballistic missile systems are involved. Both of these systems are road-mobile and can be deployed with no heavy construction work for launch pads, etc. Both the SS-3 and SS-4 are single stage vehicles which will carry a 3,000-pound warhead to a maximum range of 700 nautical miles and 1,100 nautical miles respectively…

Although we cannot be sure, it seems likely that the bulk of the personnel and equipment were shipped from the USSR as an integrated road-mobile unit, suitable for field deployment. The time required to reach operational readiness could thus be quite short. Assuming that the necessary fuelling and handling equipment is available, that communications are being installed and that warheads are in Cuba or en route, an operational MRBM capability could probably exist in Cuba within the next few weeks.

The Soviet leaders’ decision to deploy ballistic missiles to Cuba testifies to their determination to deter any active US intervention to weaken or overthrow the Castro regime, which they apparently regard as likely and imminent. This estimate of US intentions prompted Moscow’s statement of September 11th, which warned that an attack on Cuba would lead to a general nuclear conflict. The Soviets presumably believe that the presence of these missiles, which they would expect would quickly become known to the US government, will significantly increase the costs and risks of any US action against the Cuban regime. They also probably believe that the missiles will reinforce the deterrent link between Cuba and Berlin, which was implicit in the September 11th Soviet statement and in subsequent private conversations. Moscow clearly is seeking to portray Berlin as a hostage for Cuba…”