U.S. President Donald Trump ordered the U.S. military on Thursday to immediately resume testing nuclear weapons after a gap of 33 years, minutes before beginning a meeting with Chinese President Xi Jinping.

How many nuclear weapons tests have there been, why were they stopped - and why would anyone start them again?

THE NUCLEAR AGE

The United States opened the nuclear era in July 1945 with the test of a 20-kiloton atomic bomb at Alamogordo, New Mexico, in July 1945, and then dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki in August 1945 to force Japan to surrender in World War Two.

The Soviet Union shocked the West by detonating its first nuclear bomb just four years later, in August 1949.

In the five decades between 1945 and the 1996 Comprehensive Nuclear Test-B

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