Ronald D. Asmus, a foreign policy analyst and former deputy assistant secretary of state who argued successfully for expanding NATO into Eastern Europe after the Cold War, died April 30 at 53.

He died of cancer at a hospital in Brussels, where he was serving as a senior official with the German Marshall Fund of the United States, a public policy institute devoted to transatlantic issues.

NATO had origins in the Cold War as an alliance against the looming threat of Joseph Stalin’s Soviet Union. But with the collapse of communist states in the early 1990s, debate raged in foreign policy circles over how NATO should engage the former Soviet satellite countries in Central and Eastern Europe.

Dr. Asmus, then an analyst at the Rand Corp. think tank, said he thought NATO should embrace those countries as new members. Extending NATO’s security arrangements to the east would help stabilize young democracies, he reasoned, and bolster the chances for peace.

In 1993, he and two Rand colleagues, F. Stephen Larrabee and Richard L. Kugler, put forth their pro-expansion argument in what became an influential article in Foreign Affairs magazine.

“Nationalism and ethnic conflict have already led to two world wars in Europe,” the authors wrote. “Whether Europe unravels for a third time this century depends on if the West summons the political will and strategic vision to address the causes of potential instability and conflict before it is too late.”

The article was a sensation among Europe’s foreign policy establishment and helped kickstart the policy fight in America.

Those who disagreed with Dr. Asmus — including the eminent diplomat and historian George F. Kennan — said expanding NATO would serve only to push the Cold War’s stark borders farther east, raising Russia’s ire and offering no measurable security improvement.

“I think it’s fair to say that there were more people against it than were for it,” said Strobe Talbott, deputy secretary of state at the time and now president of the Brookings Institution. “What most of the arguments came down to was the Russians didn’t like it, which was true — they didn’t like it and don’t like it to this day.”

The Foreign Affairs article helped convince Talbott to bring Dr. Asmus into the State Department as a lead adviser on NATO enlargement. Dr. Asmus held that position from 1997 until 2000.

Pro-expansionists won over a wide range of allies, including President Bill Clinton and Speaker of the House Newt Gingrich (R-Ga.), who made NATO enlargement part of his 1994 Contract with America. Soon the prevailing view, Talbott said, was that “we couldn’t give Russia a veto over the right of what were now independent countries to join the alliance.”

In 1998, the U.S. Senate voted 80-19 to ratify a treaty allowing Poland, Hungary and the Czech Republic to become part of NATO.

In 2004, NATO was enlarged again with the addition of a wave of Northern and Eastern European countries including Slovakia, Estonia, Latvia and Lithuania. Albania and Croatia joined in 2009.

Dr. Asmus, who directly assisted many of those countries with their negotiations over NATO membership, was recognized with awards from the U.S. State Department and from European governments.

According to Daniel Fried, a senior U.S. diplomat who was ambassador to Poland in the late 1990s, Dr. Asmus helped provide the intellectual underpinnings for an effort that began under Clinton and continued under George W. Bush — “a bipartisan endeavor,” Fried wrote in an e-mail, “that started as an isolated, unpopular idea, ended up as U.S. and NATO policy, and is seen today as so successful that few can remember that it was once considered controversial.”

Ronald Dietrich Asmus was born June 29, 1957, in Milwaukee to immigrants from Germany. While attending the University of Wisconsin at Madison, he traveled to Berlin, which was then divided into the communist-controlled east and democratic west.

“The reality — complete with barbed wire, armed towers manned by soldiers with guard dogs and orders to shoot to kill — was a pivotal experience that changed my life and future career path,” Dr. Asmus wrote in his insider’s chronicle of NATO’s expansion, “Opening NATO’s Door” (2002). “Simply put, it horrified me.”

At Wisconsin, he switched his major from engineering to history and international relations. He later received a master’s degree and a doctorate in European studies at Johns Hopkins University’s School of Advanced International Studies.

Dr. Asmus joined the German Marshall Fund in 2002 as a senior transatlantic fellow in Washington. He later became executive director of the Fund’s Brussels office and in 2010 published “A Little War That Shook the World,” a book about Russia’s 2008 invasion of Georgia.

Survivors include his wife, Barbara Wilkinson, and their son Erik, both of Brussels; his mother, Christine Schroeder Wittke of Raleigh, N.C.; and two brothers.