Madison thought the most important clause in the Constitution was the requirement that Congress declare war. They had seen the negative consequences of monarchs who brought nations into war unilaterally, killed massive numbers of people and bankrupted countries.
Obama criticized Bush for doing this — although Bush consulted Congress a lot more than Obama has not only in Libya but the CIA led war in Pakistan. These are very dangerous wars that can easily become quagmires. Wars are unpredictable. Maybe Obama will be right and we will be out in a few days, but Lincoln thought the civil war would take six weeks, and Cheney thought the U.S. would be welcomed in Iraq with flowers. So, we’ll see how it unfolds. So far Obama has unilaterally spent several hundred million with each Tomahawk Cruise Missile costing $1.5 million.
The House of Representatives is supposed to be responsible for starting the appropriations process, we are cutting basic services (even in Obama’s crummy budget) but for war Obama (and in the end Congress) can always find the money.