What Is This Going To Accomplish?

From Newsday: The U.S. dispatched an elite group of Marines along with two warships to Tripoli on Wednesday following a mob attack that killed the U.S. ambassador to Libya and three other Americans. U.S. officials are investigating whether the violence — initially blamed on an anti-Islamic video — was a terrorist attack planned to coincide with the anniversary of 9/11.
Tuesday's stunning attack on the American Consulate in Benghazi poses a daunting task for U.S. and Libyan investigators: searching for the culprits in a city rife with heavy weapons, multiple militias, armed Islamist groups and little police control.  http://www.newsday.com/news/world/u-s-sends-marines-warships-to-libya-after-attack-kills-u-s-ambassador-chris-stevens-3-others-1.3994462
So, we are sending the in the Marines and some ships.  Why?  Who are we going to attack? 
Further, we didn't send in the Marines when Libya was in their civil war, so why now?
This just seems to be a temper tantrum thrown by Obama administration over the death of the Ambassador in Libya.
If we are going to attack, we better do it right and get the unknown thugs who killed the Americans.
Otherwise, what's the point?