From World War II until the end of the Vietnam War, the CIA and NSA were meeting their missions of providing strategic intelligence. The U2 program was the most famous project that both agencies used to great effect. The program showed the true state of Russia's Air Force and provided strategic intelligence about its nuclear capabilities. Based on the photos obtained on the over-flights we determined that Russia did not have as many bombers and fighter aircraft as had been previously thought. It also revealed how they designed their missile facilities that were later identified in Cuba before they became operational.
During World War II, the CIA inserted spies into Germany to gather intelligence on the ground. They also relied on resistance groups in the occupied countries for intelligence. This was easier during World War II since many people in the US were second-generation immigrants from Europe and still spoke their parents' language. This is far more difficult for the CIA now since most of our intelligence needs are focused in the Middle East. Most people in the CIA employment do not look like people from that region and do not speak the languages. The Israeli intelligence service does a far better job of inserting spies and informants into other countries in the region.
With the end of our war in Vietnam, there was discussion of the future "brushfire wars." These were viewed as the future of real shooting wars that would be local in scope. They were typified by the various insurrections in Africa. Russia and China used the Vietnam War as a surrogate war in which they could fight against the US through their Vietnamese surrogates without any cost to themselves other than material.
The military and CIA inserted intelligence teams into North Vietnam