There is the first flicker of hope that peace may be restored to South Sudan: it began with an acceptance of responsibility. Three factions of the South Sudanese ruling party, meeting in the Tanzanian city of Arusha, on Monday accepted that they are jointly responsible for the war that has devastated their new nation over the past 10 months.

“The parties acknowledge a collective responsibility for the crisis in South Sudan that has taken a great toll on the lives and property of our people,” says the text of a deal signed on behalf of President Salva Kiir and his sacked former deputy, Riek Machar.

The two factions, which have been at daggers drawn since the conflict erupted in December 2013, were joined by a third group. These are senior leaders of the ruling party, the Sudan People’s Liberation Movement (SPLM), imprisoned by the president for months after the fighting began, but not allied to Machar.

Together these factions make up the majority of the movement that fought for the best part of five decades to win independence from Sudan in July 2011. Faultlines, long evident within the SPLM, are now openly acknowledged. So too is their impact on the country.

“A divided SPLM will automatically fragment the country along ethnic and regional faultlines,” the Arusha document says, calling for “genuine and honest dialogue that puts the interest of the people and the nation above all”.

It is the honesty and the measure of contrition exhibited by the party’s leadership that gives some prospect that an agreement may hold this time. Previous deals have swiftly collapsed.

Another reason for cautious optimism is the intense international pressure on the South Sudanese to settle their differences. The Tanzanian ruling party is behind the initiative. An initial dialogue, held in Arusha between 12 and 18 October, is said to have gone well. But the process has much wider support. South African President Jacob Zuma’s African National Congress has been actively involved, as have the Ethiopians. The US and UK have given their backing, and Finland’s former president, Martti Ahtisaari, has been trying to bring the parties together.

Despite this intense effort there is no guarantee the initiative will not collapse. The distrust and anger inside the SPLM is deep and visceral. Kiir is seen as indecisive and has been criticised for surrounding himself with advisers who have been too keen to try to win favour with Sudan. “He wanted to end the conflict with the north – marginalising former close confidants in an attempt to get closer to Khartoum and [Sudanese] President Omar al-Bashir,” one insider told the Guardian.

There is even less trust in the former vice-president, Machar. No South Sudanese has forgotten his part in the Bor massacre of 1991, when 2,000 people died. Nor have they forgiven Machar for deciding to make his peace with Khartoum. He was rewarded with the north’s support, and become an assistant to Bashir in 1997.

In the current crisis, there have been repeated allegations that the north is supporting Machar’s rebels in their conflict with their former allies in Juba. Leaked minutes of an August meeting between Sudan’s most senior military and security officials suggested that arms supplies for Machar were very much on the agenda.

Yet peace in South Sudan will not be achieved without the support of both Kiir and Machar. The crisis has resulted in the slaughter of thousands – often on ethnic lines. South Sudanese politics is infinitely complex, but without reconciliation between the Nuer, many of whom look to Machar for leadership, and the Dinka, from whom Kiir draws much of his support, nothing will be achieved.

The outline agreement would see power shared once more between these two men. Kiir would remain president and Machar would become prime minister. The question is how their powers would be divided – and the peace process may still collapse over this.

The other key issue is what role there would be for the third group: the former detainees. Many of them are among the most senior party leaders, including Pagan Amum. He is reported to have been deeply critical of the Arusha peace process.

Even if these obstacles can be overcome, it would only result in healing the divisions inside the SPLM. There are still the wider peace talks that are being held in the Ethiopian capital, Addis Ababa. These involve the other South Sudanese political parties as well as civil society organisations. The Addis talks were adjourned this month, probably to allow the party negotiations in Tanzania to get under way.

There is much at stake. While their leaders meet and talk, South Sudan’s citizens face an acute crisis. A million or more will need food aid, having fled from their homes and having lost this year’s planting season. Aid agencies have warned that unless peace can be achieved the country faces a famine.

The talks in Arusha are a vitally important window of opportunity, but no more than that.