Complex sentence: The battle started on the morning of the 9th, the Saxon/Russian army was approximately 19,000 men strong and under the command of the saxon generalfeldmarschall Adam Heinrich von Steinau.
Split into simpler sentences: The battle started on the morning of the 9th. The Saxon/Russian army numbered approximately 19,000 men, under the command of the Saxon Generalfeldmarschall Adam Heinrich von Steinau.