The Battle of Mu'tah was triggered when a messenger Prophet Muhammad sent to Ghassani Shurahbil was murdered. This was a declaration of war.
While Prophte Muhammad himself didn't participate in this battle, thanks to the treaty of Hudaybiyya he was able to send 3000 Muslims to fight without compromising the securit of Medina.
As the army departed, the Prophet reminded them to maintain taqwa, keep to treaties kept, and to not attack women or children. He also insturcted them to offer the enemy three different surrender conditions
As they marched closer to the enemy, the Muslims realized that despite their large numbers, the opposing army outnumbered them more than 3 to 1.
Lecture notes available at http://www.why-quran.org/?p=661.
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