The Irish regiments had not been paid for over a year, and Parliament promised to fund the arrears of any soldier who joined them. In return for swearing loyalty, Grenville received his wages, and was appointed Lieutenant-General of Horse in the Army of the South Eastern Association, led by his former colleague, William Waller. However, in March 1644, he defected to the Royalists, who put him in charge of the siege of Plymouth; he also participated in the victory at Lostwithiel in September.