One of the most stunning geopolitical transformations in recent years has been the warming relationship between South Korea and China. Yet Beijing is putting those ties at risk in a fit of pique over Seoul's decision to participate in the Terminal High Altitude Area Defense (THAAD) anti-missile system. American policymakers are enjoying the spectacle of the China pushing Seoul back toward the U.S.

Beijing's relationship with South Korea began inauspiciously, with China backing North Korea's Kim Il Sung after his campaign to conquer the South in June 1950 faltered.

Although combat was halted by an armistice in July 1953, formal peace never came. Throughout the Cold War China backed North Korea. Only in 1992 did China and South Korea finally initiate diplomatic relations.