The news that President Robert Mugabe has resigned in Zimbabwe is a pivotal moment in the southern African nation’s history.

Mr. Mugabe is the only leader Zimbabwe has known since it achieved independence in 1980. Formerly known as Southern Rhodesia, the country was ruled by Britain and later by a white-minority government that was shunned by the international community.

But under Mr. Mugabe’s party, ZANU-PF, a once-wealthy nation saw its economy steadily deteriorate, and the governing party became consumed by infighting.