If the euro is preserved and Europe moves toward a more unified future, Mrs. Merkel will probably win the lion’s share of the credit, perhaps one day being hailed as Europe’s savior. But if her prescriptions turn out to be inadequate, she could reap the blame for presiding over the collapse of the euro, with untold consequences for the world economy.

Either way, Mrs. Merkel, a steely champion of austerity and fiscal discipline, seems to have assumed the nickname of her 19th-century predecessor Otto von Bismarck: the Iron Chancellor.