- Kerry sworn in, first trip likely to include stops in Egypt and Israel
- Clinton cites Turkey attack as example of challenges in the world
- Clinton visited 112 countries and flew almost 1 million miles in four years
- At 65, Clinton faces questions about whether she'll run for president in 2016
A final meeting with the president. A farewell address to the State Department staff. A terrorist attack at the U.S. Embassy in Turkey.
Hillary Clinton's last day as secretary of state on Friday seemed to be a microcosm of her four globe-trotting years as America's top diplomat.
The former first lady had a full schedule, as usual, meeting in private with President Barack Obama at the White House and then attending the send-off ceremony with clapping, cheering workers at the agency she led.
Those events were clouded by a suicide bombing earlier in the day in Ankara that killed the attacker and a Turkish security guard at the embassy. No Americans were killed or injured.
Clinton traveled nearly 1 million miles and visited 112 countries in her quest as secretary to promote better understanding of the United States and its role in the world.
However, her tenure will also be remembered for last September's attack on a diplomatic compound in Benghazi, Libya, that killed Ambassador Christopher Stevens and three other Americans.
Amid the good wishes and emotions of Clinton's farewell, Friday's bombing in Turkey was a grim reminder of the continuing threats and challenges to U.S. diplomatic efforts.
"Of course we live in very complex and dangerous times as we saw again just today at our embassy in Ankara," Clinton told the State Department ceremony.
"I know the world we are trying to help bring into being in the 21st century will have many difficult days," she added. "But I am more optimistic today than I was when I stood here four years ago."
Standing in front of a sign that said "Thank You," Clinton told the staffers jamming the area that she would miss them and the important work they do around the world.
"I will probably be dialing ops just to talk," she said to laughter.
Clinton sent her resignation letter to Obama a few hours before her successor, John Kerry, was sworn in on Friday afternoon.
"It has been an honor to serve in your administration and to represent our country around the world," the letter said.
"I am proud of what we have accomplished together on behalf of the American people and in pursuit of our interests and values. And I am more convinced than ever in the strength and staying power of America's global leadership and our capacity to be a force for good around the world," she said.
In what was billed as Clinton's farewell speech on Thursday at the Council on Foreign Relations, she said the United States needs to build "smart power" in a world where the levers of influence are changing rapidly.
American diplomats have to work with entrepreneurs and activists, use new technological tools such as social media and update the global and regional institutions that have dominated international affairs since World War II, Clinton said.
"We need a new architecture for a new world -- more Frank Gehry than formal Greek," she said. "Now some of his work at first might appear haphazard, but in fact, it's highly intentional and sophisticated. Where once a few strong columns could hold up the weight of the world, today we need a dynamic mix of materials and structures."
Clinton called the United States "uniquely well-positioned" to help construct that metaphorical global edifice.
"Those things that make us who we are as a nation --- our openness and innovation, our diversity, our devotion to human rights and democracy --- are beautifully matched to the demands of this era and this interdependent world," she said.
Kerry, who served nearly 30 years in the Senate, stepped down Friday ahead of his swearing-in as the nation's 68th secretary of state.
He was chairman of the Senate Foreign Relations Committee and the Democratic nominee for president in 2004, when he lost to incumbent President George W. Bush.
Kerry expects to take his first official overseas trip in coming weeks, according to a U.S. official who said stops would likely include Israel and Egypt.
Clinton, 65, now faces persistent questions about whether she will mount a second White House run in 2016 to break perhaps the highest remaining glass ceiling.
"Well I'm not thinking about anything like that right now. I'm looking forward to finishing my tenure as secretary of state and catching up on 20 years of sleep deprivation," Clinton said Tuesday at a global "town hall" forum in Washington.
Observers note her answer was a less emphatic denial than previous responses that seemed to rule out any possibility of reentering the political fray.
According to a CNN/ORC International poll conducted last month, 85% of Democrats and independents who lean toward the Democratic Party said they would be very or somewhat likely to support Clinton if she sought the Democratic nomination.
Clinton was considered a favorite in 2008, but Obama, who served with her in the Senate, got an early jump in the primaries. Clinton was unable to capture the momentum in a fierce political battle.
Despite the rugged campaign, Obama chose Clinton for his Cabinet and she embraced the role.
In a joint interview with CBS that aired on Sunday, Obama said that he believes Clinton's legacy will rank among the best.
"It has been a great collaboration over the last four years," Obama said. "I'm going to miss her."