most people put the credit in Reach. It actually started on a small agricultural planet called Harvest. a type of locating device the Covenant use to locate objects of the Forerunners (objects they consider holy) marked a human ship as holding these valuables. they then scanned the planet and founds the planet sprawling with them. these objects were in fact humans therefore tying us with the forerunners. this infuriated three san'shyuum- the soon to be prophets- these three were afraid that, if the rest of the covenant were to find out the truth of the humans, their power would be uprooted. they therefore took over the title of the three current prophets and declared war on humans. maybe it helped and sorry it was so long.