Complex sentence: The Ambala Cavalry Brigade had mobilized in August 1914 as 3rd (Ambala) Cavalry Brigade and served on the Western Front until it was broken up in March 1918.
Split into simpler sentences: The Ambala Cavalry Brigade had mobilized in August 1914 as 3rd (Ambala) Cavalry Brigade. It served on the Western Front until it was broken up in March 1918.