President Barack Obama and Greek President Prokopis Pavlopoulos review the Presidential Guard in Athens, Greece, Nov. 15, 2016. President Obama arrived in Greece for the first stop of his final foreign tour as president and the first visit to Greece by a sitting U.S. president since Bill Clinton in 1999.

Police and demonstrators clash during a protest against the visit of President Barack Obama in Athens, Greece, Nov. 15, 2016. Obama is in Athens for talks with the country's political leadership before heading to Berlin as part of his last major trip abroad.

Greek Prime Minister Alexis Tsipras escorts President Barack Obama upon his arrival for a meeting at the Maximos Mansion on Nov. 15, 2016 in Athens, Greece. On the first leg of his last foreign trip as leader, Obama stressed that a strong Europe was "good for the world and the U.S."

Demonstrators shout slogans during a protest against the visit of the U.S. president in Athens, Greece, Nov. 15, 2016. President Barack Obama arrived in Athens for a two-day official visit.

President Barack Obama tours the Acropolis with Dr. Eleni Banou, director of Ephorate of Antiquities for Athens, Ministry of Culture, Nov. 16, 2016 in Athens, Greece.

President Barack Obama walks with Professor Dimitrios Pandermalis, President of the Acropolis Museum, during a tour of the museum, Nov. 16, 2016 in Athens, Greece.

President Barack Obama tours the Acropolis Museum with Professor Dimitrios Pandermalis, president of the Acropolis Museum, Nov. 16, 2016 in Athens, Greece.

President Barack Obama delivers a speech at the Stavros Niarchos Foundation Cultural Center, Nov. 16, 2016 in Athens, Greece.

President Barack Obama arrives at Tegel Airport for his visit to Germany on November 16, 2016 in Berlin.

German Chancellor Angela Merkel and President Barack Obama have a private dinner at the Hotel Adlon at the beginning of his three-day visit to Berlin on Nov. 16, 2016 in Berlin.

German Chancellor Angela Merkel greets President Barack Obama upon his arrival at the Chancellery on Nov. 17, 2016 in Berlin.

President Barack Obama passes the Brandenburg Gate while walking from the U.S. Embassy to the Adlon Hotel, Nov. 17, 2016 in Berlin.

President Barack Obama and German Chancellor Angela Merkel address a press conference after their meeting at the chancellery in Berlin on Nov. 17, 2016.

From left, Italian Prime Minister Matteo Renzi, French President Francois Hollande, President Barack Obama and German Chancellor Angela Merkel talk before a meeting with European Union leaders at the German Chancellery, Nov. 18, 2016 in Berlin. European leaders headed to the German capital to seek assurances from President Obama on the course that Donald Trump might take, given the populist mogul's pledges to shake up the world order.

President Barack Obama says goodbye to German Chief of Protocol Jurgen Mertens before his departure, Nov. 18, 2016 at the Tegel airport in Berlin. President Obama's choice of Berlin as the stop for his European farewell tour has been interpreted by some observers as the passing of baton of the defense of liberal democracy to German Chancellor Angela Merkel.

President Barack Obama walks across the tarmac with Second Vice President of Peru Mercedes Araoz, center, during his arrival at Jorge Chavez International Airport in Lima, Peru, Nov. 18, 2016.

President Barack Obama speaks during a Young Leaders of the Americas Initiative town hall meeting at the Pontifical Catholic University of Peru, Nov. 19, 2016 in Lima, Peru.

President Barack Obama listens to a question during a Young Leaders of the Americas Initiative town hall meeting at the Pontifical Catholic University of Peru, Nov. 19, 2016 in Lima, Peru.

President Barack Obama greets the audience after speaking at a town hall with Young Leaders of the Americas Initiative (YLAI) in Lima, Peru, Nov. 19, 2016.

Along with members of their delegations, President Barack Obama, right, and China's President Xi Jingping, left, meet during the Asia-Pacific Economic Cooperation (APEC) in Lima, Peru, Nov. 19, 2016.

Wearing a Peruvian shawl, President Barack Obama poses for a family photo at the APEC Summit in Lima, Peru Nov. 20, 2016. In the front row from left, Japan's Prime Minister Shinzo Abe and South Korea's Prime Minister Hwang Kyo-Ahn, in back row with Obama is Thailand's Deputy Prime Minister Prajin Juntong, left and and Vietnam's President Tran Dai Quang, right.

President Barack Obama shares a laugh with Canada's Prime Minister Justin Trudeau and Chile's President Michelle Bachelet at the opening session of the Asia-Pacific Economic Cooperation (APEC) summit in Lima, Peru, Nov. 20, 2016.

President Barack Obama speaks during a press conference on the last day of the Asia-Pacific Economic Cooperation (APEC) Summit in Lima on Nov. 20, 2016. Asia-Pacific leaders vowed to fight protectionism at the close of a summit upended by President-elect Donald Trump's victory and his attacks on free-trade deals.

President Barack Obama talks with Russia's President Vladimir Putin at the opening session of the Asia-Pacific Economic Cooperation (APEC) in Lima, Peru, Nov. 20, 2016.

President Barack Obama boards Air Force One for departure at the conclusion of his last foreign trip as President, at Jorge Chavez International Airport Nov. 20, 2016 in Lima, Peru.