 Signs of impending communist action did not go unnoticed among the allied intelligence collection apparatus in Saigon . During the late summer and fall of 1967 both South Vietnamese and U.S. intelligence agencies collected clues that indicated a significant shift in communist strategic planning . By mid @-@ December , mounting evidence convinced many in Washington and Saigon that something big was underway . During the last three months of the year intelligence agencies had observed signs of a major North Vietnamese military buildup . In addition to captured documents ( a copy of Resolution 13 , for example , was captured by early October ) , observations of enemy logistical operations were also quite clear : in October , the number of trucks observed heading south through Laos on the Hồ Chí Minh Trail jumped from the previous monthly average of 480 to 1 @,@ 116 . By November this total reached 3 @,@ 823 and , in December , 6 @,@ 315 . On 20 December , Westmoreland cabled Washington that he expected the Viet Cong and North Vietnamese " to undertake an intensified countrywide effort , perhaps a maximum effort , over a relatively short period of time . " 
