Ever since the beginning of this republic, there has been a dynamic tension between the power of the Senate and the power of the executive branch in foreign policy making and the declaration of war. Congress has sought to roll back some of the executive's power in the face of grave mistakes (particularly in Vietnam and Iraq) through legislation like the War Powers Resolution. Congress has also tried to constrain wartime budgets or set timetables for withdrawal. Therefore, Congress can exercise considerable power in foreign policy if it chooses.
So why does Congress fail to insist that it have a say in military action? The answer is simple. Much of Congress is too busy giving the President support to insist on constitutional guarantees. Even without a declaration of war, the United States went to war in Vietnam and Iraq with a broad majority in Congress. The loudest voices supporting intervention in Libya came from Senator John Kerry (Chairman, Senate Foreign Relations Committee) and Senator John McCain (ranking member, Senate Armed Services Committee), drowning out lonely voices in the minority like Senator Richard Lugar (ranking member, Senate Foreign Relations Committee).
The executive should have broad power to shape foreign policy and protect the United States. But Senate oversight should be compulsory in cases where the executive branch wants to take military action without a pressing threat to national security or vital interests. This does not mean that the United States should never act without such interests, but it does mean that the Senate should retain its role as a check and balance in this great constitutional system. Further, the Senate should insist on protecting minority rights and hearing out those few Senators with legitimate concerns about military intervention.
Also see: http://lugar.senate.gov/news/record.cfm?id=332967&&