The Rev. Joseph Musser’s family has always lived in the region of Alsace, but not always in the same country.

His grandfather fought for the Germans in World War I, and his father for the French in World War II. Today, no one is fighting anymore. His great-niece lives in France but works in Germany, crossing the border her ancestors died fighting over without even noticing it.

It is this era of peace and borderless prosperity that champions of the European Union consider the bloc’s singular achievement.

“The foundation of the European Union is the memory of war,” said the Reverend Musser, 72. “But that memory is fading.”