Prophet Muhammad sent a mission to Yemen to invite them to Islam. The first mission was led by Khalid ibn Al-Walid, who was a harsh man who had fought against the Muslims in the Battle of Uhud, but he had an aristrocratic background and by making him feel valued Prophet Muhammad had hoped to turn him into a productive member of the Muslim community.
However once in Yemen, instead of following the prophetic principles of dawah Khalid hastily resorted to violence. The Tribe of Numayr, feeling threatened, sent a delegation to the Prophet in Medina to ask for protection from Khalid.
The Prophet sent Imam Ali to Yemen to do damage control and instructed Khalid's men to follow him instead. While sending Imam Ali he gave him various bits of advice, emphasizing the need to display wisdom, to soften people's hearts, and to remain upright.
The prophet emphasized the importance of avoiding bloodshed if possible, to the point where he said to not even engage in battle unless they kill one of your men first, and to even then first give them a chance to repent.
Lecture notes available at http://www.why-quran.org/?p=754.
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