According to Gordan Chang, President Eisenhower's top priority was to break apart the Sino-Soviet monolith, so in his 1963 memoir The White House Years he barely mentioned Sino-Soviet tensions, so as “to avoid saying anything that could hinder the emergence of the Sino-Soviet split.”1 Under President Johnson, the split in the Communist bloc gradually deepened. A Top Secret CIA report from 22 February 1965 cited an unnamed Soviet source as saying “that Kosygin's trip to Hanoi will result in the Soviets giving ‘defensive’ aid to North Vietnam in the form of fighter planes, SAMs and radar equipment.”2 The CIA reported that on 28 March 1965, a three-way agreement among the USSR, China and North Vietnam had been reached to transport arms shipments across China by railway. Coincidentally, a declassified report of a sunken Soviet cargo ship carrying missiles to North Vietnam might explain why these Surface-to-Air Missiles (SAMs) had to be brought by railway through China (see Document 7).

Transporting missiles by train required China's cooperation. Beginning in 1965 the Soviet government felt obliged to provide North Vietnam with SAMs. The first SAM site was spotted from the air on 5 April 1965, located 15 miles southeast of Hanoi, but there were as yet no missile equipment there. However, this did not “mean the end of Sino-Soviet friction on this issue.” In particular: “Moscow has shown itself reluctant to date to ship extensive aid to North Vietnam by sea, and recent Soviet allusions to an ‘American blockade’ suggest that the USSR fears a repetition of Khrushchev's disastrous backdown in the face of the US naval quarantine in the Cuban Missile Crisis. Since both air and land transit of significant quantities of Soviet military aid to the DRV would almost certainly necessitate passage over or through China, there remain ample opportunities for further difficulties and mutual recriminations.” In fact, China was placing strict limits on the transit of Soviet personnel, and Presidium Member Suslov said that “although the Chinese had agreed to let Soviet nationals go through China by rail, they had ‘changed their minds several times in this regard’.”