Asmus was born to a family of German immigrants who came to Milwaukee, Wisconsin after World War II. He grew up in Milwaukee and Mequon, Wisconsin graduating from Homestead High School. Asmus was an early and strong proponent of expanding NATO into Eastern Europe. He was U.S. Deputy Assistant Secretary of State for European Affairs in the Clinton Administration from 1997 to 2000. He played an important role in the 1999 Washington summit of NATO, when Poland, the Czech Republic and Hungary joined the Alliance. He was also a senior analyst and fellow at Radio Free Europe, RAND and the Council on Foreign Relations. For his diplomatic work, Asmus was decorated by the US Department of State and the governments of Estonia, Georgia, Italy, Latvia, Lithuania, Poland, and Sweden.
Asmus held a PhD in European Studies, a Master’s degree in Soviet and East European studies from the Paul H. Nitze School of Advanced International Studies of the Johns Hopkins University, and a BA in political science from the University of Wisconsin–Madison. He was the Executive Director of the Brussels-based Transatlantic Center and was also responsible for strategic planning at the German Marshall Fund of the United States.
Asmus authored Opening NATO's Door: How the Alliance Remade Itself for a New Era (Columbia, 2002), about the push to open NATO to Eastern European countries, and A Little War that Shook the World (Palgrave Macmillan, January 2010), about the conflict between Russia and Georgia in 2008. Being one of the most persistent advocates for the integration of Georgia into the European Union and NATO, Asmus viewed the conflict in terms of a larger Russia–West relations and argued that it was Georgian independence, and its Westward orientation, which angered Russia and set the groundwork for war.
Asmus died of a lung cancer-related illness in Brussels on April 30, 2011. The U.S. Department of State, governments of the Baltic states and Georgia expressed their condolences over the death of Asmus.