How solid is the West? At last weekend’s Munich Security Conference, the world’s largest gathering of security policy makers and officials, the theme was “Westlessness,” referring to the sense of disorientation that many Europeans feel in this age of America First.

Since the 1940s, U.S. leadership in the service of a united and secure Europe has been the one unchanging feature in the Continental landscape. For generations, the U.S. committed to protect Europe from Russia, maintain bases in Germany to prevent it from threatening...