On a sultry mid-August evening last week, world currency markets were rocked by what amounted to nothing short of a regime change: China announced a devaluation and freer float of its currency, the yuan or renminbi.

The move recalled an episode more than four decades earlier, on Aug. 15, 1971, when President Richard Nixon stunned the world by unilaterally severing the dollar’s last link to gold. Like the U.S. action at the time, China’s was portrayed as a measured, one-time adjustment that amounted...