Wars of the Three Kingdoms
As the struggle over religious practice and royal authority intensified between Charles I and his Scottish opponents, both sides began preparing for a possible civil war. In December 1637, Charles made Ruthven Muster Master General for Scotland, and in 1639 appointed him Governor of Edinburgh Castle. The outbreak of the Bishops' Wars in March prevented him taking up this position, while he turned down command of the Scottish Royalists, as he was made subordinate to his less experienced social superiors. The First Bishops' War ended in June with the Treaty of Berwick, but was seen as only a temporary pause in hostilities.
Ruthven was finally able to enter Edinburgh Castle in November 1639, the garrison containing a number of experienced Scots officers who had served in Germany, including his son Alexander. When the Second Bishops' War began in April 1640, they held out until September, when forced to surrender by starvation. One of the few bright spots in a disastrous war for Charles I, the right to use the Ruthven name was restored to the Ballindean branch of the family in May 1641, while Ruthven was made "Earl of Forth" in March 1642.
When the First English Civil War broke out in August 1642, Ruthven was appointed Marshal General of the Royalist forces. On the eve of the Battle of Edgehill in October, he had a furious row over how to arrange the army with his immediate superior, the Earl of Lindsey. Lindsey resigned, and later died of wounds received fighting with his regiment, while Ruthven was appointed Lord General in his place. In the subsequent Battle of Brentford, a brigade led by Ruthven stormed the town, but the Royalists failed to take London after being repulsed at Turnham Green, and withdrew to Oxford.
As Lord General, Ruthven acted as Charles' Chief of Staff, and played an active role, being wounded at the Siege of Gloucester and First Battle of Newbury in 1643. He informally commanded the Royalist army at Cheriton in March 1644, a defeat that ended hopes of retaking South-East England from Parliament. Although largely caused by the indiscipline of his subordinates, this impacted his reputation at court, especially given the increasing prominence of younger, more aggressive leaders like Charles' nephew, Prince Rupert of the Rhine. Ruthven was also showing signs of age, Royalist statesman Clarendon later noting that while "a man of unquestionable courage and integrity", by 1644 he "was much decayed in his parts, and with long...immoderate drinking, dozed in his understanding...".
At 72, Ruthven himself recognised he was too old for active service, and in May 1644 he was created Earl of Brentford in the Peerage of England. However, in early 1644, Charles ordered Prince Rupert to relieve York, an expedition that ended in defeat at Marston Moor on 2 July. In his absence, Ruthven played a key role in the Royalist campaign in Western England, beating off one Parliamentarian army at Cropredy Bridge in June, then trapping another at Lostwithiel in September, arguably the most comprehensive Royalist victory of the war. Returning to Oxford, he was wounded at Second Newbury on 27 October, and on 6 November resigned his position in favour of Prince Rupert.