1643, he served in the first Siege of Basing House, and was slightly wounded in the Battle of Alton on 13 December. Less than a week later, he was shot in the stomach in an assault on Arundel Castle, allegedly surviving only because the cold weather stemmed the flow of blood. Birch recovered from his wounds in time to fight at Cheriton in March 1644, a lesser known Parliamentarian success that forced Charles I onto the defensive in South East England. At Cropredy Bridge in June, he commanded the rearguard that held the bridge long enough to allow Waller's main force to retreat. Shortly after this, Birch raised a regiment of infantry which was shipped to Plymouth to reinforce the garrison and spent the rest of the war in South West England and the Welsh Marches. Although not part of the New Model Army, his unit took part in its 1645 Western Campaign including the capture of Bridgwater and Bristol. On 17 December 1645, Birch led a surprise night-time attack on the Royalist garrison of Hereford, which had recently repulsed a month-long siege by Scots Covenanters, and the Royalist commander Barnabas Scudamore was later accused of betrayal. Appointed Governor of Hereford, Birch also fought at Stow-on-the-Wold in March 1646, the last major battle of the war, and captured Goodrich Castle in June, just before the war ended. However, victory resulted in increasingly bitter disputes over the post-war political settlement between radicals within the New Model like Oliver Cromwell, and the moderate party in Parliament, which included his former commander William Waller, and Denzil Holles. At the same time, Parliament's desperate economic position made reductions in the military a matter of urgency. Birch became involved in a struggle with the other regional Parliamentarian commander, Edward Harley, as both men sought to ensure their troops were the ones retained. Coming from one of the most prominent members of the landed gentry in Herefordshire, and one of the few families to support Parliament in 1642, Harley defeated Birch in 1646 when he stood as MP for Herefordshire. Despite this setback, in September Birch was elected for Leominster, but under the Self-denying Ordinance was required to give up his military commission. Appointed High steward of Leominster in 1648, he also invested heavily in purchasing church lands, which made him extremely wealthy. Disputes over a peace settlement with Charles I and religious policy split Parliament