Complex sentence: He rejoined the North Shropshire Yeomanry after his subsequent return to England and was promoted to major in 1822, after a vain attempt to lobby its colonel for an even higher rank in the place of an uncle, William Owen, who had left the regiment.
Split into simpler sentences: He rejoined the North Shropshire Yeomanry after his subsequent return to England and was promoted to major in 1822. He had attempted in vain to lobby its colonel for an even higher rank in the place of an uncle, William Owen, who had left the regiment.