Via ShrinkWrapped:

What many people don’t know is that the Joint Chiefs offered President Johnson a plan to quickly defeat North Vietnam. They wanted to cut all railroads into the country, mine and blockade Haiphong Harbor, and, if necessary, bomb the irrigation dams on the Red River. Their reasoning was that with no access to outside goods and material and no rice crop, the North would have had to come to the bargaining table in earnest. President Johnson refused to follow their advice.

The mistake that President Johnson made was to think he could use limited warfare to put enough pressure on Uncle Ho to agree to that idea. He was worried about starting a larger war with the Chinese and Russians if we went all out in Vietnam. This may have been a valid worry. However, if the North had been isolated quickly and their rice irrigation system destroyed, they would have had little choice but to negotiate in good faith. Had this been accomplished quickly the Chinese and Russians may have accepted it as a fait accompli ala their decision on North and South Korea.

In my opinion it is always a mistake to try to prosecute a limited war.