Actually based on what we thought we knew at the time then a case can be made for a president going into Iraq for nation security purposes with or without congressional approval. Then In the first gulf war we had Iraq invading Kuwait and saudia Arabia and yet we waited mths to give Iraq time to withdraw and the world to unite behind the cause.
In libya there is no national security issue. There is absolutely no rationale for not getting congressional approval.
When you say "what we thought we knew" are you referring to the WMD issue? If so, then let's remove it from the equation and list only the irrefutable evidence to make the case.
1. Ignored 17 UN resolutions over 12 years – the original being a cease-fire agreement to end the Gulf War.
2. Murdered 100s of 1000s of his own people – some using WMD
3. Had terrorist training camps set up in his country – see Salman Pak
4. Paid $25,000 each to the families of the so-called pali suicide bombers
5. Provided safe haven for known al-Qaeda terrorists
6. Invaded another sovereign nation
7. Conducted the Oil for Palaces scandal – which France and Russia were into neck deep
8. Continuously fired at US and GB planes enforcing the no-fly zone
9. Kicked the UN weapons inspectors out of the country