Following World War II, the U.S. alliance with Western Europe stood as the cornerstone of American foreign policy in the face of Cold War threats from what was then the Soviet Union. Forged in the North Atlantic Treaty Organization (NATO) -- the alliance's enduring defense pact -- the partnership is rooted in the shared values of democracy, rule of law and free-market principles. But with the emergence of China and India as global economic powers, the Arab Spring revolutions and Iran's uncertain nuclear ambitions, the United States has shifted its political and security priorities to the Asia-Pacific region, leaving Europe worried that its historic ties with the United States are fraying. In May, President Obama will host two meetings of European leaders that could help define the trans-Atlantic alliance for years to come: a NATO summit in Chicago and a summit of the Group of 8 industrialized nations at Camp David, the presidential retreat.

Roland Flamini is a Washington-based correspondent who writes a foreign-affairs column for CQ Weekly. Fluent in six languages, he served as Time magazine's bureau chief in Rome, Bonn, Beirut, Jerusalem and the European Common Market and later served as international editor at United Press International.