The Cuban crisis highlights the crucial part played by intelligence gathered either by agents (HUMINT), listening devices (SIGINT), or photography (PHOTINT).
As early as 1961 the National Security Agency had been monitoring messages from and to Soviet ships en route to Cuba or secretly unloading military equipment (tanks, radars, anti-aircraft guns). Then, the CIA found out that Il-28 bombers and MiG fighters were arriving.
More materiel was shipped in 1962. The CIA spotted 57 ships in the month of August. By then, their conclusion was that the Cubans were building up an anti-aircraft defence system to Soviet standards. Confirmation came from the CIA that launch sites were under construction. Their surface-air SAM-2 missiles could destroy aircraft flying at high altitude. USS Oxford, fitted with listening devices, patrolled the Cuban zone in order to intercept radio communications. The NSA detected the presence of air-traffic controllers speaking Spanish with strong Slavic accents.
And yet the CIA persisted in thinking that the Soviets were not prepared to run the risk of a clash by deploying nuclear warheads in Cuba.
But converging information kept coming from other sources, such as the French Intelligence services. At McCone's suggestion, Philippe Thyraud de Vosjoly, the top SDECE (French Intelligence) man in Washington D.C., whose eventful life inspired the Léon Uris bestseller Topaz (published in 1967), was sent to Cuba on a mission in 1962, and reported the presence of medium range missiles on the island. At the same time, the French Ambassador in Havana, Roger du Gardier, was reporting nightly landings of Soviet troops wearing Cuban militia uniforms, and the unloading of launch materiel. Colonel Houel informed the Americans that major earthworks were under way in the north of the island, which a US Air Force spy-plane soon confirmed.