The Campaign of Dhat Al-Salasil occurred soon after "defeat" at the Battle of Mu'tah, when various Ghassanid affiliated tribes sensed an opportunity to invade Medina. They thought the Muslims would be weak and demoralized.
A bedouin learned about the gathering army and informed Prophed Muhammad, who designated an army and commander to go fight them.
From here the Shia and Sunni narratives diverge sharply. Sheikh Azhar Nassers explains both the different views and shares ideas for why they may have ended up with such drastically different versions.
Lecture notes available at http://www.why-quran.org/?p=669.
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