Why? because when mohammed started preaching about Islam, the Quraish tribes trowed him out of his home (Makka) and he had to migrate to madina... Till this time Muslims were not allowed to take up war with any one as it was not instruected by Allah. but then Allah instructed them to go out for a war (even though you are less in numbers) but I will protect you and help you to win it. Those who belive in me will be firm and those who don't will run away like cowards...
The battle happened, and muslim won the war against 1000 quraish. This was the first battle fought by muslims against quraish.
Prophethood came to mohammed at the age of 40 and god instructed to pick up a war only after 13 years of Mohammed prophethood (his age was 53 by then).
Islam doesn't preach volience, but islam also doesn't preach injustice.
