Mr. Roberts, a historian, is author most recently of The Storm of War: A New History of the Second World War (Harper, 2011).

Queen Elizabeth II acceded to the British throne 60 years ago this week. The Commonwealth will mark the occasion with celebrations throughout the year, but many republicans claim that the queen has made no genuine difference to the history of her country. Is that so?

The past six decades haven't been easy for the United Kingdom, and were it not for the monarchy there is no telling what social and political unrest might have dominated. To celebrate the good times properly, it's important to recall the perilous ones, and what might have happened had the queen not been, in the words of the historian John Grigg, "a bastion of stability in an age of social and moral flux."

When the queen's father, King George VI, died in February 1952, Evelyn Waugh remarked that his reign had been the most disastrous since that of King Stephen in the 12th century. Appeasement, World War II and the transfer of power in India meant that Britain had slipped from great-power status—where Churchill could meet Stalin and Roosevelt on equal terms at Tehran and Yalta—to the position of a second-division power teetering on bankruptcy. Elizabeth II's reign saw Britain further shorn of any delusions of global grandeur and forced to readjust to a new, less exalted but more honest place in the world. That this was managed without severe internal dissension was largely due to the political stability and continuity personified by the queen.

In France, the decolonization of Algeria led to bloody riots on the streets and several attempts to assassinate President Charles de Gaulle. The British were spared such traumas as the queen visibly supported the post-imperial settlement...