The whole question of when the President needs Congressional authorization to send troops is a pretty murky one. Constitutionally, the power to wage war rests with the Congress. But the President is Commander in Chief of the armed forces, charged with the responsibility of responding to any "clear and present danger" to the US (there's a murky phrase for you -- I'd rather try to reach consensus on "what is folk?"). Our elected officials sometimes try to clarify this through legislation (the War Powers Act being perhaps the most pertinent recent example), but more often they prefer to leave the definitions fuzzy so they can decide what they want to do with respect to any particular issue on political and party grounds. Often our Congressional representatives prefer to wait and see how an operation turns out before they express an opinion; they initially dodge the question by saying that "while our troops are under fire, we think it's important to stand behind the President," while still reserving the right to object after the fact.

It's worth noting that every President who has been subject to the War Powers Act has objected to it as an unconstitutional encroachment on the powers of the Executive Branch, and none has stated that he considers himself bound by it in all cases. It's also worth remembering that, before we mobilized on a large scale in Vietnam, President Johnson sought and obtained sweeping authorization from Congress through the infamous Tonkin Gulf resolution. So we should not assume that the Congress will necessarily keep the President in check when it comes to military actions.