The upcoming election in Saxony-Anhalt could give the far-right Alternative für Deutschland control of a state government for the first time in Germany’s postwar history. Its rise offers a crucial lesson for democracies elsewhere: closing ranks against extremist parties is no substitute for effective governance.

NEW YORK—On May 8, 1654, the mayor of Magdeburg appeared before the Imperial Diet at Regensburg with two hollow copper hemispheres and 30 horses. Nothing joined the metal halves, yet the horses could not pull them apart. The hemispheres were held together by the vacuum within and the weight of the atmosphere around them. Only when a valve was opened and air rushed in did the spheres fall apart.