In the Constitution, the Founders intentionally prohibited the Executive branch from having the power to unilaterally determine whether or not the country would engage in war. Few were more adamant about this than James Madison, the "Father of the Constitution," who wrote:
"The constitution supposes, what the history of all governments demonstrates, that the executive is the branch of power most interested in war, and most prone to it. It has accordingly with studied care vested the question of war in the legislature."
Thus, Congress has the power to determine if the country will wage offensive war and against whom. Once that decision is made by the Congress, the President is in charge of waging that war.
The power in question is delegated to Congress in Article I, Section 8, Clause 11 of the Constitution:
[Congress shall have Power...] To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water;
As I wrote in an early-2007 article, Article II, Section 2 of the Constitution, on the other hand, refers to the President as the "commander-in-chief of the army and navy of the United States." What the founders meant by this clause was once war was declared, it would then be the responsibility of the President, as the commander-in-chief, to direct the war.
Alexander Hamilton supported this when he said that the President, while lacking the power to declare war, would have "the direction of war when authorized."
Thomas Jefferson stated this quite eloquently when, in 1801, he said that, as President, he was "unauthorized by the Constitution, without the sanction of Congress, to go beyond the line of defense."
Thus, under the Constitution, the President, acting without a Congressional declaration of war, is authorized only to repel invasion and sudden attacks. Pre-emptive strikes, "humanitarian missions," and other undeclared military expeditions are not powers delegated to the executive branch in the Constitution, and are, therefore, unlawful.
As an aside, it's also important to note that no federal branch has the constitutional authority to transfer powers delegated to it to another branch. So, for example, if Congress would pass a resolution giving the President the power to make the final decision on whether or not the country will go to war, that would be a transfer of delegated power, and unconstitutional as well.
There simply is no debate. Congress, not the President, decides if the country will go to war.
Confronted with the Constitutional requirement that Congress is the federal branch that determines when the country goes to war, supporters of unilateral executive power will often take one of two paths to avoid following the Constitutional mandate that Congress declare the war before the Executive can take action.
1. Refer to the action as defensive.
This is the classic argument that "proves too much." As James Madison pointed out, a constitutional argument is "triable by its consequences." Here, the consequences would be essentially unlimited executive war power, since almost any significant activity can be linked to "national security" or a need to "defend American interests." Because it is incontrovertible that unlimited executive war power is not what the Founders' Constitution granted, the argument fails.
The argument is also a scary one, because historically "national security" often has been used to excuse the suspension of individual rights.
In short, actions only qualify as "defensive" under the constitution if they are in response to a direct attack or an imminent threat of attack. While not conclusive, there is Founding-era evidence to support the constitutionality of a defensive military response to protect U.S. personnel abroad as well. It's also instructive to note that even this broader understanding is limited to "U.S. personnel abroad" and not merely "U.S. interests."
2. Refer to the action as something other than "war."
Under the Constitution, a war is a war whether you call it a war or something else.
Constitutional scholar, Rob Natelson, wrote about the legal meaning of the word "war"" in March, 2011:
Founding-Era dictionaries and other sources, both legal and lay, tell us that when the Constitution was approved, "war" consisted of any hostilities initiated by a sovereign over opposition. A very typical dictionary definition was, "the exercise of violence under sovereign command against such as oppose." (Barlow, 1772-73). I have found no suggestion in any contemporaneous source that operations of the kind the U.S. is conducting were anything but "war." [emphasis added]
All U.S. military actions qualify as "violence under sovereign command." And attacks, whether for strategic, political, or humanitarian purposes, are always "over opposition."
The last time Congress constitutionally-declared war was on December 11, 1941; against Germany, in response to its formal declaration of war against the United States. This resolution was quickly accomplished with a statement that was well-under one page in length; yet it still clearly delineated who the enemy was, and what was to be done. Three days earlier, and one day after being attacked at Pearl Harbor, Congress declared war on Japan with a similar clarity. Both actions resulted in a clear-cut military victory.
The short version? Unless fending off a physical or imminent invasion or attack, the president is required to get a Congressional declaration of war before engaging in military hostilities in another country.
Since it's unlikely that the executive branch will limit its own power, and there's very little evidence that congress will use the power of the purse to do so either, it's going to be up to the people of the states to make that happen whether the feds want us to or not.