For an old Cold Warrior the scene on a bright October afternoon was surreal: America’s Second Cavalry Regiment crossing a Romanian river on a Soviet-built tactical bridge assembled by the Romanian Army, while overhead Vietnam-era MiG 21s carried out mock attacks, with German-made antiaircraft guns manned by Romanian crews simulating the destruction of the intruding MiGs.

The symbolism of the river crossing brought home to me the precarious condition of the U.S. military presence in Europe. American armor crossed on Romanian...