Joseph Lagu, a military officer who led southern Sudan’s war for independence from Sudan in the 1960s and who, after negotiating autonomy in 1972, became one of Sudan’s top political leaders and diplomats, died on Tuesday in Juba, the capital of South Sudan. He was 94.

Salva Kiir, the president of South Sudan, which gained full independence in 2011, announced the death in a statement and called for a period of national mourning.

Sudan, the third-largest country in Africa by area, gained its independence from Egypt and Britain in 1956. Power in the country rested with its Arabic-speaking Muslim population in the north, which outraged the darker-skinned Africans of the south.

An independence movement sprang up almost immediately in the south, and soon broke out into open warfare. An estimated 500,000 to one million people died in the nearly 15-year war that followed.