Initially, Nixon was ambivalent about the publication, because it was McNamara/Johnson policy. However, Kissinger and other advisors told him it would set bad precedent to have national security documents leaked in such a manner. It's a complete joke the issue was ever made about Nixon.
Odd how they were decidedly less interested in a similar situation with James Risen of the NYT and CIA analyst Jeffery Sterling being prosecuted under Obama just a few years ago.