Riek Machar Teny, the vice-president of South Sudan who is planning to oust the president, is known to some for his marriage to a British aid volunteer, Emma McCune, who went to work in Sudan in 1987 during the civil war.

At the time Machar was a commander in the Sudan People's Liberation Movement/Army (SPLM/A) fighting the government in Khartoum. After they married, McCune threw in her lot with the southern rebels. In 1993, while pregnant with Machar's child, she was killed in a car crash in Nairobi. Her story was later told in a book, Emma's War, by Deborah Scroggins.

Machar's career as a guerrilla leader brought similar vicissitudes.

His quest for independence for South Sudan led him into conflict with the then SPLM leader, John Garang, who initially favoured a reformed but united Sudan in which the equal rights of non-Arabs and non-Muslims would be respected.

In 1991 Machar broke away, forming a rebel faction, the SPLM/A-Nasir.

A period of complex, rival alliances based on ethnic and tribal lines followed, involving sometimes heavy fighting between Machar's forces and the SPLA. His groups received help from the Sudanese government and in 1997, along with other disaffected rebels, he signed the ill-conceived Khartoum Peace Agreement and became an assistant to President Omar al-Bashir.

Machar eventually patched up his differences with Garang and rejoined the SPLA as a senior commander in 2002. He was subsequently involved in negotiations leading to the 2005 Comprehensive Peace Agreement (CPA), which ended the civil war, and after Garang's death in an accident he became South Sudan's first vice-president on independence in July 2011.

Friction between Machar and President Salva Kiir has built up in the past two years as South Sudan's new SPLM-led government has struggled to meet the daunting challenge of building a nation state from scratch. Observers in the capital, Juba, say memories of Machar's machinations during the war and the many deaths that resulted from the split with Garang have left a residue of distrust.

In his interview with me, Machar was highly critical of Kiir's record as president and urged him to step down, thereby avoiding a leadership contest. But he also made plain that he was ready for a fight, at the same time revealing his knowledge of British political precedents - he holds a PhD from the University of Bradford, England.

“Even in your own country, Margaret Thatcher had to leave after leading the Conservative party for a very long time. Tony Blair also had to leave after winning three consecutive elections and give way to the next generation,” he said. Theirs was an example that Kiir would do well to follow, he suggested. He rejected any comparison between himself and Gordon Brown.

His criticism of Kiir aside, Machar offered several reasons why the new government was underperforming. It was difficult, he said, to make the switch from guerrilla movement to governance; Khartoum continued to make problems for the impoverished new state, particularly over oil exports; and South Sudan was the world's youngest independent country. It would take time to build institutional strength, he said.

“After independence, the expectations of the people shot up very high. They want us to turn this country into another Dubai or Korea or Malaysia, countries that have moved fast in their economic and social development ... That is good, but we have to weigh and measure those expectations against the reality on the ground ... We haven't met the expectations of the people in the last two years.”

Machar rejected a suggestion the ruling party elite had become out of touch with ordinary people. “We are of the people, we come from the people. We are pastoralists, camel-herders, peasants. It would be difficult to say we are out of touch. We live with them.”

By arrangement with the Guardian