And by "opening lines," we literally mean lines, not sentences. Because the first sentence is five paragraphs long.

This resolution is a decision, by Congress, about what to do with Vietnam. Therefore, it needs to state a goal. The first line states that goal by saying:

To promote the maintenance of international peace and security in southeast Asia […] (1)

Clear enough, and quite patriotic. The purpose of whatever we are about to read (spoiler: giving the President unlimited power to wage war) is for peace and safety.