Rules of Engagement (ROE) refer to acceptable applications of military power during a conflict or operation. They are meant to provide military commanders and their actions are being conducted within a defensible framework by creating a clearly defined set of rules and options. Things such as when military force is authorized, what evidence must be gathered in order for certain responses to be allowed and so forth.
From 1962 until 1965, the U.S. greatly revised and expanded the original rules of engagement for Southeast Asia. Moving from a strictly advisor role in the early 60’s to a supporting combat role by the mid 60’s, the revised ROE’s approved new missions and parameters for U.S. troops and advisors, especially after the Tonkin Gulf incident. Once congress has passed the Tonkin Gulf Resolution, it allowed military commanders greater freedom in rewriting the ROE’s.
Johnson’s role as Commander in Chief gave him ultimate authority over the ROE’s, but he usually deferred to his national security team when changes had to be made.