Riek Machar, the South Sudanese rebel leader, returned to the country Tuesday after fleeing in 2013 and was sworn in as vice president. His deal with President Salva Kiir is aimed at ending the two-year civil war that had engulfed the world’s newest nation.

Here are the circumstances that led to the fighting, from the BBC:

Mr Machar fled Juba at the start of the civil war in December 2013.

He had been accused of trying to organise a coup, which he denied - but it set off a round of tit-for-tat killings, which developed into a full-blown conflict.

The fighting split quickly along ethnic lines. Machar is from the Nuer ethnic group, the country’s second-largest, while Kiir is Dinka, the largest group. The conflict has spawned a massive refugee crisis, pushing many of those fleeing the fighting to Ethiopia’s Gambela region, which abuts South Sudan.

That region was the scene last week of a bloody conflict involving South Sudanese tribes, this time between the minority Murle and the Nuer.

On April 15, Murle tribesman raided 13 villages in the Jakawa area of Gambela. Witnesses told Al Jazeera that the fighters, who were armed with Kalashnikov rifles and dressed in army fatigues, launched simultaneous, well-organized attacks on villages where the Nuer and Anuak ethnic groups live. Some 208 people were killed in the raids, dozens wounded, and 108 children abducted, Getachew Reda, Ethiopia’s communications minister, told Reuters. In response, Ethiopian troops conducted cross-border operations last week into South Sudan, with the government’s approval, to rescue the abducted children.

The April 15 raid was the latest incident in the generations-long fighting between the Murle and Nuer.

For generations, the tribes have launched retaliatory attacks for cattle, land, and water rights—and revenge. The Los Angeles Times provides some context, in this article from 2012:

The Murle minority of about 148,000 is often casually derided by other South Sudanese as "the problem" because of the tribe's history of trying to boost its numbers by abducting children of rival tribes.

But U.S. anthropologist Jon Arensen, who spent eight years living with the Murle in the 1970s and 1980s, says its members are often demonized in South Sudan.

The two sides fight differently: Murle men move like guerrillas in small bands, attacking, stealing cattle, abducting children and retreating. The Nuer move in huge, heavily armed columns of many thousands, pouring into enemy villages with devastating power.

The Los Angeles Times’s Robyn Dixon blamed the latest violence on a “long-standing security vacuum” in South Sudan. The country has virtually no infrastructure that could adequately preempt or defend such a large-scale raid, especially since most Murle-Nuer conflicts occur in remote villages, far removed from access to help.

The border between Ethiopia and South Sudan is porous and “occasional local outbreaks of violence in this area” are to be expected, David Shinn, a professor at George Washington University in Washington, D.C., and the former U.S. ambassador to Ethiopia, told Al Jazeera.

Still, wrote Kelsey Lilley in AllAfrica, the cooperation between South Sudan and Ethiopia over the abductions is a “positive sign,” but she acknowledged that such interethnic conflicts, intensified by civil war, “know no borders.” What follows the rescue efforts and the nascent peace deal in South Sudan will be more telling, and ultimately more important, for the world’s newest nation.