"President Kiir has been drawing on those memories and referencing them, even in his public statements," Kumar said. "So, we know that that is something that is certainly salient on his mind as well as the minds of many other South Sudanese."
Machar has publicly apologized for any role in the Bor massacre. Kumar says the two men initially tried to put their differences behind them to form a unity government.
"Riek Machar was brought in as vice president for the new South Sudan with the hope that he would be able to bring his significant power and constituency, both military and political, to the table to ensure that there would be a more cohesive national governing force," she said.
But the government's unity attempt has faltered. On VOA's Press Conference USA John Campbell, who is a veteran U.S. diplomat who is now at the Council on Foreign Relations, said it appears individual agendas have replaced unity.
"Very often, behind ethnic conflict, you will find various political figures that seek to exploit ethnic identities in order to advance their own particular political agenda," he said. "And, I think there is at least some of that underway now in South Sudan."
Ahmed Soliman, a Horn of Africa researcher at London's Chatham House, says Kiir's current problems with leadership have evolved beyond his rift with Machar and now reflect the changing nature of the government.
"An opening up of the party to enable challenges within the leadership would always, I think, entail some conflict. And, we may be seeing some of that now," he said.
Soliman says South Sudan's leadership is also beginning to feel the pressures of generational change, which may include moving away from the military-style leaders who ushered the nation into existence.