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tions to the neglect of guerrilla or the People's
War approach: Thanh had failed to coordinate
operations with the revolutionary elements - the
guerrillas and local political functionaries. His
critics favored a protracted revolutionary struggle
and advocated a return to a "defensive" posture of
small-unit strikes. Thanh, in a speech before
COSVN in mid-1966, contemptuously dismissed
this approach. "If we want to take defensive positions,
we should withdraw to India." 26
(D) Despite Thanh's bravado, he did relent
and accepted some limits to his earlier strategy.
He was willing to admit that adaptations had to
be made in tactics when fighting the Americans.
He allowed for additional forms of political struggle
to supplement the military actions. He limited
large unit operations to the northern part of
South Vietnam, near the DMZ. Regular PAVN
units and large PLt\F formations engaged U.S.
Army and Marine units near places like Con
Thien, Camp Carroll, and Khe Sanh. Other fights
started outside Danang and Hue. In the Central
Highlands, isolated special forces bases were hit.
North of Saigon, communist units battled numerous
Allied battalions that were trying to clear out
the communists' complexes in sanctuaries along
the Cambodian border.
(U) Thanh had not abandoned his big-unit
strategy. He just changed the way the main force
units engaged the Americans, which was referred
to as the way "a tiger leaps at his prey." 27
Communist units would attack isolated bases and
units and then disappear into the bush when the
large American units arrived on the scene.
Despite this change in tactics, by the middle of
1967, the communists again had taken horrific
casualties and had little to show for it. Their units
were still in the field, but nothing had been
denied the Americans.
(8/t~I) A major factor in the string of communist
military failures was the growing capabilities
of the American SIGINT effort in South
Vietnam. In the three years of direct U.S. combat
involvement, American SIGINT developed in
pace with the growth of the Vietnamese communist
communications.
(8//81) Prior to 1965, Vietnamese communist
operations could be characterized as "guerilla" in
that the final planning was left to lower echelon
units, which, in turn, seldom used radios to conduct
militray operations. D/F and analysis in this
period was effective in locating units in their
enclaves or bases, seldom on the move. After
1965, as larger regular PAVNformations took the
field, the communications profile of communist
military operations changed. Certain C3 patterns
of behavior appeared that tipped off their actions.
These patterns, recovered by SIGINT analysts,
allowed them to determine with a great degree of
accuracy, the operational intentions of communists
units. So effective was this analysis that,
after 1965, probably no major communist military
operations went undetected.
(S//Sf) These patterns were termed "SIGINT
indicator" by the cryptologists in Vietnam and
NSA, and were used to warn Allied commanders
in Saigon and the field of communist moves. By
late 1967, analysts had developed five major categories
of SIGINT indicators that tipped off communist
military activity These were:
1. Changes to Signal Operating Instructions
(SOl). These features included introduction of
new tactical callsigns, procedural signals,
increased communications scheduling, unscheduled
changes to the SOl, and a failure to implement
a regularly scheduled SOl change. Of particular
interest was the appearance of so-called
"watch nets" in communist communications.
Watch nets basically acted as a 24-hour "call up"
by which units could contact and set up transmission
schedules. This technique provided flexibility
for units on the move that might encounter
unexpected difficulties.
2. Communications Network Structural
Changes. These entailed the activation of unit for-
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