Complex sentence: They joined at that site the Sixth Maine Infantry, Mott's Battery and a company of cavalry, but by August 12, the 2nd Vermont Infantry and the 33rd New York Infantry had joined them.
Split into simpler sentences: They joined at that site the Sixth Maine Infantry, Mott's Battery and a company of cavalry. By August 12, the 2nd Vermont Infantry and the 33rd New York Infantry had joined them.