Eyeing the ruin of Venezuela under 21st-century socialism, one can easily forget that Hugo Chávez came to power in 1999 on a pledge to root out corruption. Forty years earlier Fidel Castro’s Cuban revolution derived much of its popular support from widespread disgust with the corruption of the Batista regime.

This might seem like ancient history, but it helps explain why the unconstitutional dissolution of the Peruvian Congress by President Martín Vizcarra last week has the region’s democrats on edge.

Peru...