Complex sentence: It had already proved its usefulness during the Luddite riots of 1811 and 1812, and continued to be used as mounted police, most famously, the Manchester and Salford Yeomanry were largely responsible for 11 deaths and 400-500 injuries in the Peterloo Massacre of 1819.
Split into simpler sentences: It had already proved its usefulness during the Luddite riots of 1811 and 1812, and continued to be used as mounted police. Most famously, the Manchester and Salford Yeomanry were largely responsible for 11 deaths and 400-500 injuries in the Peterloo Massacre of 1819.