Now that the US has initiated fighting in Libya, there have been muted grumblings about whether Obama should have at least consulted with Congress before using the military. After all, the Constitution says that it is Congress that declares war, not the President. Such grumbling would have more force if Congress were in fact more in control of the process of making war. In fact, however, the President has been in control from the beginning, starting with Washington's dispatch of Jay to negotiate with the British without consulting with Congress. When the House protested its exclusion from the process, it lost, and has never since tried to be a part of the process.
The attempt to limit the President's ability to make war is perhaps the weakest and most flawed part of the Constitution, allowing the rise of the Imperial President. It was not as much of a problem at the beginning, because the President literally did not have an arm to control, but this did not last.
The Founders implicitly gave the President almost complete authority over the conduct of foreign relations and the military, and their feeble, ambiguous attempt to restrict this authority by directing that Congress got to declare war, has simply not succeeded. The problem is that the President can direct the armed forces to go and do whatever he wants, and until he calls it a war, Congress is just not involved. Thus the President can involve us de facto in a war, and then confront Congress with a fait accompli, where Congress has no choice but to go along.
The President is able to engage in gunboat diplomacy without declaring war, or start a war and then get the agreement of Congress. This has been true from the beginning. Jefferson fought the Barbary pirates without a declaration of war. Monroe acquired the Florida territory with the help, deliberate or accidental, of Andrew Jackson when he invaded Florida while chasing Indians. Polk provoked Mexico into attacking us, and then got Congress to declare war on Mexico. This pattern has been repeated over and over again, such as in the Spanish-American War, the Korean War, and the Vietnam War. Even in the two world wars, by the time the wars were declared, the President had already deeply involved the country.
With the permanent establishment of the armed forces after WWII, the ability of the President to control the country through the provocation of war has become even more dangerous. The excuse of a foreign enemy and national security becomes an irresistible tool for domination of even domestic policy.