First English Civil War
Following the outbreak of the Irish Rebellion in October 1641, Brereton supervised the transport of troops and supplies from Chester to Ireland. When the First English Civil War began in August 1642, this made him an obvious choice for commander of Parliamentarian forces in the area. However, most of the Cheshire gentry was Royalist, including his cousin, William, Baron Brereton, and he was forced to withdraw to London.
Along with a number of other regions, at the beginning of the war some prominent Cheshire notables attempted to remain neutral, and in December 1642 issued the Bunbury Agreement, which demanded an end to fighting in the area. This proved impossible to enforce and in March 1643, Parliament sent Brereton back to Cheshire with 500 men, where he linked up with 2,000 local volunteers. Despite lacking military experience, he soon proved an energetic and resolute commander, winning two minor but significant victories at Middlewich and Hopton Heath. Establishing his headquarters at Nantwich, he soon attained superiority over Arthur Capell, Royalist commander in Shropshire, Cheshire, and North Wales.
Over the next few years, Brereton waged an aggressive and relentless campaign throughout the North Midlands, one of his key subordinates being Colonel Robert Venables, a long-time family connection and friend, who later served in Ireland. Their activities forced the Royalists to divert resources from other areas, as Chester was essential for funnelling men and material from their supporters in Ireland and North Wales.
In October 1643, Capell was replaced by Lord Byron, who assembled an army of over 5,000, many of them veterans from the war in Ireland. Defeated at Second Middlewich in December, Brereton appealed to Sir Thomas Fairfax for support. At Nantwich in January 1644, their combined force routed Byron, who lost over 1,500 men, most of his artillery, and baggage train and spent most of the next two years blockaded in Chester.
At Malpas in August 1644, Brereton added to his reputation by routing Royalist cavalry under Marmaduke Langdale who had escaped from the defeat at Marston Moor. Combined with his influence in the region, this meant Brereton was one of the few to be exempted from the February 1645 Self-Denying Ordinance, under which army officers could not also be MPs. Chester finally surrendered in February 1646, and in March Brereton fought in the last major battle of the war at Stow-on-the-Wold. He then besieged Lichfield, one of the last Royalist strongholds in England, which capitulated in July 1646.
More than 2,000 of his letters from this period survive, and are one of the most important sources for understanding Parliamentary administration during the war. They also provide insights into its internal politics; Brereton was a member of the 'War Party', those who viewed military victory as essential before any negotiations with Charles. The 'Peace Party' included Denzil Holles, one of the Five Members whose failed arrest in January 1642 was a major step on the road to war.