Presidents take us into armed conflicts using the War Powers Act...then Congress falls in line...every time. It's a total giveaway that the Founding Fathers never would have wanted. They didn't want the President to have the power he has now.
To be fair, the founders lived in a different time. A time when you usually had months of warning leading up to a war.
In todays world as part of his job Defending this nation and it's interests. the President is often presented with Time Sensitive Issues. It is not practical to expect him to get approval from the Full Congress before acting all the time. Sometimes if you do not act now, you might as well not act at all.
Now that said, Obama had 3 weeks leading up to this attack on Libya in which it was becoming more and more clear we might have to act. Why he did not go to congress until Friday night is beyond me.