A Blockade of the Vietnamize Coastline
The biggest challenge for the Vietcong at the time was being able to supply their gorillas in the south. For decades Hanoi had been smuggling supplies south via the ocean with an improvised fleet of junks, trawlers, and freighters.
Source: exploreindochina.com (Left) At any given time there were anything from three to six thousand trucks operating on the Trail. (Right) They were a prime target.
That is until the U.S. Navy blockaded the Southern coastline of Vietnam. The Vietcong would now have only one rout to provide logistical support.