Complex sentence: By September 1914, the cavalry division was numbered the 1st Cavalry Division, and joined by the 2nd Cavalry Division which had been formed on 6 September, and the 3rd Cavalry Division formed on 29 September.
Split into simpler sentences: By September 1914, the cavalry division was numbered the 1st Cavalry Division. It was joined by the 2nd Cavalry Division, which had been formed on 6 September, and the 3rd Cavalry Division, formed on 29 September.