Complex sentence: An additional battalion, the 22nd Rough Riders, was then formed in 1901, when the 20th Battalion was disbanded and the new regiment was formed from Boer War veterans.
Split into simpler sentences: An additional battalion, the 22nd Rough Riders, was formed in 1901 from the 20th. After the 20th Battalion was disbanded in 1901, a new regiment was formed from Boer War veterans.