Even if there had been a battalion of marines in ships five miles away Obama wouldn't have attempted a rescue. He was afraid of violating Libyan sovereignty and airspace. And to get authorization from a marginally functioning Libyan government might have taken days. Obama was afraid any armed U.S. incursion into Libya could have unraveled, possibly embarrassing him and angering the Arab masses. This during the campaign season.
A decision was made to cut losses and write off the consulate and the annex before the fight was even over. For the last five years the foreign policy decisions—including Benghazi—coming out of the White House are not shaped by Obama’s foreign policy experts but by his political advisers.
IMHO, that analysis is incomplete. The same person didn't mind directly intervening to have cruise missiles launched to kill Qaddafi, without any notification to Congress or any declaration of war, and bragged about specifically targeting that family.