Ron D. Asmus, a former U.S. diplomat who took charge of the Brussels office of the German Marshall Fund of the United States in 2005, has died aged 53.

The fund, a think tank that promotes greater cooperation between North America and Europe, said on its website that Asmus died Saturday from cancer.

As U.S. deputy assistant secretary of state for European affairs from 1997 to 2000, Asmus was part of talks that saw formerly communist East European nations join the NATO alliance. He wrote a book about that — "Opening NATO's Door" — in 2002.

In 2010 he published "The Little War That Shook the World," a book about the 2008 war between Russia and Georgia.

Asmus became executive director of the fund's Brussels office in 2005 after three years as senior trans-Atlantic fellow at the think tank's Washington head office.

A native of Milwaukee, Wisconsin, he held degrees in Soviet and European studies.

He is survived by his wife, Barbara Wilkinson, his son Erik, his mother, Christine Schroeder Wittke of Raleigh, North Carolina, and two brothers, Peter Asmus of Stinson Beach, California, and Jeffrey Asmus of Plano, Texas.