Hasan was acknowledged at Kufa in 661 and his position collapsed within months, not on a battlefield but through desertion. The army sent toward Syria lost men steadily, the vanguard commander went over to Muawiya with a large part of it, and Hasan himself was attacked and wounded in the thigh at Mada'in by men of his own camp. Negotiation followed from a position that could not be defended.
The sources preserve the terms in several versions, and no two lists agree exactly, which is the first thing to say about them. The elements that recur are an amnesty for his father's partisans across Iraq and the Hijaz, an end to the public cursing of Ali from the pulpits, payments from the treasury, and a provision that Muawiya was not to nominate his own successor but to leave the choice to the community after his death.
Every one of those provisions was later reported as broken, and the last of them most conspicuously, since Muawiya did nominate his son Yazid, which is the direct occasion of Karbala.
The assessment divides along confessional lines but not neatly. Twelver theology holds that an imam's decision to make peace has the same authority as an imam's decision to fight, and that Hasan's treaty and Husayn's stand are two applications of one office to different circumstances. Sunni tradition, reading a hadith that gives the caliphate thirty years, counts Hasan's months as its completion and treats the abdication as the act that preserved the community. Both readings are attempts to make an unwelcome outcome intelligible, and the surviving record of the terms is thin enough to support either.