On 16 January 1643, Thomas, Lord Grey of Groby was appointed commander-in-chief of the midland counties association and then ordered to take special care of Nottingham during the second Civil War. He took up headquarters there in June 1643, with a force of approximately 6,000 men. In a letter to his son Thomas, dated 5 March 1643, Henry Grey (Thomas' father) describes a battle to sweep the country, going through such towns as Lutterworth, Hinckley, Barwell, Lichfield, and Newark. On 29 August 1643, at Aylesbury, he joined the Earl of Essex on the march to relieve Gloucester. After the siege was raised, he fought at the first battle of Newbury for which he received thanks. In 1644, he received more appreciation for the reduction of places in Derbyshire. He then left Leicester due to a misunderstanding but was, in 1645, petitioned back to meet a royalist attack. In 1648, Lord Grey raised troops in Leicestershire. After the defeat of the Scots at Preston, he pursued the Duke of Hamilton and his horse to Uttoxeter and took credit for his capture, though Duke Hamilton claimed he surrendered. Until August, Grey held various commands in the militia. In 1651, he was sent to raise volunteers with the commission of commander-in-chief in the counties of Leicester, Nottingham, Northampton and Rutland, to meet the Scottish invasion. In September, Sir Edward Massie surrendered to Lord Grey after the battle of Worcester.