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

What is it about the month of August? Why should we still persist in regarding it as a quiet time—with Congress in recess, business slowed down, and people on holiday—when so many world-historical events take place in this month? You can ignore the Ides of March, but history shows that it's in the dog days of August that great events take place.

Ever since the Roman Senate proclaimed in A.D. 8 that the eighth month of every year be named after the Emperor Augustus (63 B.C.-A.D. 14), nephew and adopted heir of Julius Caesar, the month has seen a disproportionate share of cataclysmic events.

In the last century alone, World War I broke out on Aug. 4, 1914, and Adolf Hitler ordered the invasion of Poland on the night of Aug. 31, 1939. World War II was only won with the dropping of two atomic bombs in August 1945. Other 20th-century conflicts sparked in August: the Vietnam War, with the Gulf of Tonkin incident of August 1964; the Russian invasion of Czechoslovakia in 1968; and the Gulf War, when Saddam Hussein invaded Kuwait in August 1990...