It is not clear that anything could have been done at the point word of the attack reached them, which is in no way meant to suggest that nothing could have been done overall as proper security should have already been been in place all along, but such a low risk operation should have been attempted even if there was only a small chance of saving the Ambassador's life.
"Ninety minutes after news of the attack reached Washington, Obama, Sec. of State Hillary Clinton, JCS Chair General Martin Dempsey, and a national security adviser convened for an oval office meeting in which they ultimately rejected the course of U.S. military intervention. Instead, they decided to reach out to the Libyan government to ask if they would send reinforcements.
When the U.S. personnel at the consulate left the main building for what was supposed to be a safe house, questions regarding the deployment of forces seemed moot.
But the battle was still raging, and Ambassador Stevens' life was close to its end.
In hindsight, anonymous officials privy to details of those Sept. 11 decisions asked why we didn't at least send aircraft from the U.S. base in Sicily -- which is less than 500 miles away from Benghazi. The line of thinking is that that such a show of force might have given the attackers second thoughts."