JUBA, South Sudan (AP) — Stepping out of a helicopter last week, South Sudan armed opposition commander Ashab Khamis came face-to-face with his rival in a crushing five-year civil war, army Gen. Keer Kiir Keer.

The meeting, witnessed by The Associated Press, was their first attempt at reconciliation since the conflict began and a crucial test of a new peace agreement ending a war that has killed nearly 400,000 people.

The governor of Wau state where the meeting occurred called it a "historic event," while the atmosphere around the table was tense. Surrounded by bodyguards, the regional command...