Complex sentence: Doyle started his military career in March 1771 when he joined the (48th (Northamptonshire) Regiment of Foot) whereupon he rose to the rank of Lieutenant in 1773.
Split into simpler sentences: Doyle started his military career in March 1771 when he joined the 48th (Northamptonshire) Regiment of Foot. He rose to the rank of Lieutenant in 1773.