After weeks of build-up, the U.S. initiated the use of military force against Libya in March ’11. Many politicians — particularly Congressman Kucinich from Ohio — have called these attacks unconstitutional in that they were ordered solely by President Obama and the Executive Branch. Congressman Kuchinich even went so far as to call it an impeachable offense. It should also be noted that in 2007 then Senator Obama stated that “the President does not have the power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.”
The correct answer isn’t immediately clear. On the one hand, under Article II, Section 2, the President is the “Commander in Chief of the Army and Navy of the United States.”  On the other hand, Congress has the exclusive power “to declare war” under Article I, Section 8 of the Constitution.
The struggle, then, is what is the relationship between Congress’s power to declare war and the President’s status as Commander in Chief? Because there are really no cases interpreting these provisions (who would sue the President or Congress in war time?), much attention will need to be given to the text of the Constitution itself.
In Article I, Congress — not the Executive — has the power to make “Appropriations” and “provide for the common Defence [sic]”; in other words, the power to provide resources for military action rests with the legislature. So, while the President may be Commander in Chief of the military, he has nothing to command except what Congress may provide. Congress could, if it disagrees with the President, simply cut off funding for a war.
But, assuming that there is funding and an existing military force with resources, what is the scope of the President’s role as Commander in Chief and when does he require congressional approval?
Both sides generally agree that the President has the inherent authority to act without Congressional approval in defense of an imminent threat to the nation. Both sides disagree, however, on the use of offensive military force.
There is good textual support that suggests that the President does not need Congressional approval before using offensive military force. Article I, Section 10, Clause 3 explicitly prohibits States from “engaging in War, unless actually invaded, in in such imminent Danger as will not admit of delay” unless they have obtained the “Consent of Congress.” No such limitation or pre-requisite is imposed on the President. No check with respect to the use of military force is found in Article II.
Furthermore, much of the debate over the power to initiate military force centers on the meaning of “declare War” in Article I. The Constitution distinguishes between “declaring” war, “engage in” war (Article I, Section 10, Clause 3) and “levying” war (Article III, Section 3, Clause I). Congress merely has the power to “declare war” — not the power to “levy” or “engage” in war.
“Declarations” of war in history have been primarily used to give notice of the legal grounds for the war — to more fully articulate to opposing nations and a country’s citizens why the war has been initiated. The Declarations were really never a pre-requisite, but more of a formality. Moreover, British history is full of undeclared wars; and because our Constitution and the terms of art articulated therein generally come from British law, it seems that the Founders were certainly apprised of the fact that a legislature’s declaration of war was not a pre-requisite to military action. (Indeed, in the history of the U.S., there have only been 5 times Congress has “declared” war: the War of 1812, the Mexican-American War, the Spanish-American War, WWI and WWII).
But, even supposing that one does challenge the President or Congress for the unconstitutional use of military force, there are other constitutional hurdles present. In O’Connor v. U.S., 72 Fed. Appx. 768 (10th Cir. 2003), the federal court dismissed a lawsuit challenging President Bush’s intention behind the war in Iraq because the plaintiff did not have “standing” (i.e., an injury in fact that was redressable by the court); and the court is not permitted to weigh the wisdom and discretion of the President’s decision-making in pursuing the use of military force. There is also the issue of presidential immunity. See Nixon v. Fitzgerald, 457 U.S. 731 (1982) (The President has absolute immunity from civil damages based on any action that the President took within his official responsibilities). Sure, you could be right that the President is acting unconstitutionally, but good luck getting in court and challenging it.
In sum, it appears that the President has almost unlimited authority over the use of the military. Congress, however, may indirectly negate and “check” this Presidential authority by cutting the purse strings of the military. In terms of the use of the military for defensive purposes, the President has the power to repel domestic threats against the U.S. In terms of the use of the military for offensive purposes, the President likely has the authority to do this because no checks exist on the Executive’s authority in Article II (like with the States) and declarations of war have primarily been used as formalities spelling out the reasons for a war.
Applying this to Libya, it would seem to me that President Obama does indeed have the authority to attack Libya…but Congress could cut the funding for the attacks.
 Many constitutional scholars contend that Baron Montesquieu was a great influence on the Founders when drafting the Constitution. Accordingly, some have traced the President’s status as “Commander in Chief,” rather than Congress or another legislative body,to Montesquieu’s writings. See Montesquieu’s The Spirit of the Laws at 165 (an army should be directly dependent on executive power, because an executive will naturally command more respect and obedience: “The army will always score a senate and respect its officers. It will not make much of the orders sent from a body composed of people it believes timid and, therefore, unworthy to command it.”)