President Barack Obama observed a moment of silence and laid a wreath this afternoon to remember the 12th anniversary of the Sept. 11, 2001, attacks, events that still weigh heavily over the United States and over his tenure as commander in chief.

The attacks, in which hijacked airliners were flown into New York's World Trade Center and the Pentagon in Washington, triggered a global fight against al Qaeda extremists and their affiliates that continues to this day.

"Let us have the strength to face the threats that endure, different though they may be from 12 years ago, so that as long as there are those who would strike our citizens, we will stand vigilant and defend our nation," the president said after laying a wreath at the Sept. 11 memorial at the Pentagon.

"Let us have the wisdom to know that, while force is at times necessary, force alone cannot build the world we seek," he said.

Before going to the Pentagon, the president observed a moment of silence on the South Lawn of the White House. With first lady Michelle Obama, Vice President Joe Biden and Jill Biden by his side, he stood still for a moment at 8:46 a.m., the moment the first plane hit the World Trade Center.

Just 12 hours earlier, Obama had given a nationally televised address to a war-weary nation about the possible need for military action against Syria if diplomatic initiatives fail get Syrian leader Bashar al-Assad to give up his chemical weapons.

At the Pentagon, the president acknowledged the decade of conflict that has followed Sept. 11. He paid tribute to the four Americans, including the U.S. ambassador, who were killed in an Islamist militant attack on a U.S. diplomatic compound in Benghazi, Libya, last Sept. 11.

"We pray for all those who've stepped forward in those years of war, diplomats who serve in dangerous posts, as we saw this day last year in Benghazi," he said.