ANDREWS AIR FORCE BASE, Md. (AP) — President Barack Obama on Friday honored the four Americans killed in an attack on the U.S. consulate in Libya, recalling their lives in deeply personal terms and declaring the United States will never pull back on its principles or “retreat from the world.”

“Their sacrifice will never be forgotten,” Obama said as four flag-draped cases rested near him. He had come to witness the return of those slain in the assault on the American diplomatic mission, including the U.S. ambassador, Chris Stevens.

In the heat of a presidential election year, the scene was a gripping reminder of the danger facing Americans in diplomatic and military service every day, and of the turmoil in an incendiary region of the world that continues to test Obama’s leadership.

Always in the background, campaign politics gave way to a sense of sheer loss. Secretary of State Hillary Rodham Clinton’s voice broke as she spoke before the president, and she appeared to be fighting tears as she listened to him.

“They knew the danger, and they accepted it,” Obama said. “They didn’t simply embrace the American ideal. They lived it.”

Americans Sean Smith, Glen A. Doherty and Tyrone S. Woods were also killed in a chaotic rush on the consulate.

Said Obama of all four men: “They embodied it: the courage, the hope and yes the idealism, that fundamental belief that we can leave this world a little bit better than before. That’s who they were, and that’s who we are. If we want to truly honor their memory, that’s who we must always be.”

The transfer of remains came three days after an attack on the consulate, one of a series of assaults on U.S. outposts in Muslim countries that U.S. officials blame on an anti-Muslim video made in the United States.

Clinton said the rage and violence aimed at American missions was prompted by “an awful Internet video that we had nothing to do with.”

The president met privately with family members of the fallen before stepping into a cavernous hangar at this base he knows well, home to Air Force One. Emerging from a C-17 cargo plane, six Marines each carried the cases on top of stands before four waiting hearses, as a color guard led the somber procession.

Clinton and Obama both spoke of how the four men lived their lives — and how their mission would go on.

“This work, and the men and women who risk their lives to do it, are at the heart of what makes America great and good,” Clinton said. “So we will wipe away our tears, stiffen our spines, and face the future undaunted.”

Said Obama: “The United States of America will never retreat from the world. We will never stop working for the dignity and freedom that every person deserves.”

Also attending the ceremony were Vice President Joe Biden and Secretary of Defense Leon Panetta.

The searing images of burning flags, breached embassies and smoldering cars have shocked the nation.

The deaths on Tuesday, the anniversary of the Sept. 11, 2001 terrorist attacks, and the anti-American sentiment behind them underscored a foreign policy paradox for Obama. Many of the protests in the Arab world were in countries that underwent Obama-backed revolutions during the Arab Spring.

Illustrating the volatility, Friday’s ceremony unfolded as a Marine rapid response team arrived in Yemen’s capital as protests erupted there.

In addition to Stevens, the ceremony also honored three other Americans killed in Benghazi — Smith, an Air Force veteran who worked as an information management specialist for the State Department; Doherty, a former Navy SEAL who worked for a private security firm and was protecting the consulate in Benghazi; and Woods, also a former Navy SEAL who had served protective duty in various U.S. posts.