Was the Vietnam War an undeclared or declared war?
By the United States it was an undeclared war.
But that really does not matter. We went into Vietnam in the early 1960s to support the government which we had played a very large part in getting put in to place. That was used as an excuse to get us involved in a conflict between North and South Vietnam that dated back to the early Eisenhower years. The other excuse, also created by the Eisenhower administration, was the "Domino Theory." That said, if we let Vietnam fall to the communists then who is next?