纪念911之际美国仍受恐怖威胁

Americans paused and reflected Sunday as the nation marked the 15th anniversary of the September 11, 2001 terrorist attacks that left thousands dead and propelled the United States into its longest-ever wars. Somber ceremonies, from New York to Pennsylvania to the Pentagon, were held against the backdrop of a presidential race in which national security is a prominent theme.

The day of remembrance featured reminders that the fight against adherents of radical Islam continues.

"What we face in the context of the global terrorist movement is a long term problem associated with social, economic and political issues across that region and the world, which has created such a widespread radicalization that one group after another has sprung up over the years," said Retired Marine Gen. John Allen on ABC's This Week program.

U.S. officials know the threat has evolved.

“We are safer when it comes to protecting against another 9/11-style terrorist-directed attack from overseas,” Secretary of Homeland Security Jeh Johnson told ABC. “We are in a new environment, however, where we have to be prepared against and try to prevent lone wolf-style attacks, the self-radicalized actor.”

The administration's critics want a more muscular U.S. posture on the world stage.

“We're sitting back waiting for the next attack. We should be on offense,” said former New York City mayor Rudi Giuliani.

“We will remember those three very famous words: peace through strength,” said Republican presidential candidate Donald Trump at a recent campaign rally. “Nobody's going to mess with us.”

But a modicum of restraint is also required, according to the administration's defenders.

“You've heard Donald Trump say he would order our troops to torture. You've heard him say he would order troops to kill family members of terrorists,” said Democratic presidential candidate Hillary Clinton late last week. “You would know that he is advocating illegal actions against our own laws as well as the laws of war.”

While a ferocious presidential contest exposes rifts in a bitterly polarized nation, the 9/11 anniversary recalls a time when Americans rallied as one.