From the time Sputnik first orbited Earth in 1957 to the fall of the Soviet Union 34 years later, Western cooperation in manned spaceflight was cemented by a common ideology and a common foe. Its capstone was the International Space Station. But today, the United States and Europe, which built the space station, have reached a crossroads as they search for ways to put astronauts on Mars. One path could lead to tighter cooperation, not only between the US and Europe, but also with Russia, China, and other nations interested in manned spaceflight. The other path could lead to an international space race in which the US may find itself still in the lead but increasingly isolated.