In a memo from Principal Deputy Assistant Attorney General Caroline D. Krass, sent, ironically, on April 1 2011, she concludes that;
The President had the constitutional authority to direct the use of military force in Libya because he could reasonably determine that such use of force was in the national interest.
Prior congressional approval was not constitutionally required to use military force in the limited operations under consideration.
The memo contains 13 pages of legal analysis that is challenging to read and references many other documents and legal cases of constituionality. About half way through the justification starts by referencing Article II of the constitution, which states that the President bears the "vast share of responsibility for the conduct of our foreign relations" and "independent authority in the areas of foreign policy and national security"