Although his father was a
Royalist, when the
First English Civil War began in August 1642, Montagu served in the
Eastern Association army led by his
Parliamentarian cousin,
the Earl of Manchester. He raised a regiment of infantry which during the 1644 campaign fought at
Marston Moor, the
Siege of York and
Second Newbury. Despite his family relationship, Montagu supported those in
Parliament who expressed dissatisfaction with the conduct of the war by Manchester and
Essex. This resulted in the creation of the
New Model Army in February 1645 and the passing of the
Self-denying Ordinance, requiring those holding military commissions to resign from Parliament. As members of the
House of Lords, Manchester and Essex were automatically removed, since unlike MPs they could not resign their titles. Montagu's regiment was incorporated into the New Model, taking part in the June 1645
Battle of Naseby, followed by the
capture of Bristol. In October he resigned from the army as required by the Ordinance when was appointed MP for
Huntingdonshire, a seat formerly held by his father who died in September 1644. Montagu played no part in the
Second English Civil War and retired from Parliament after
Pride's Purge in December 1648 to live quietly at home. He returned to politics in 1653 when his neighbour
Oliver Cromwell nominated him to the
Barebones Parliament as MP for Huntingdonshire, a seat formerly held by his father who died in September 1644. He was also appointed to the
English Council of State, an office he held until it was dissolved in 1659, and was re-elected to the
First Protectorate Parliament in 1654, then the
Second Protectorate Parliament in 1656. , painted 1666, part of the
Flagmen of Lowestoft series. During the
Anglo-Spanish War (1654–1660), he was appointed joint
General at Sea with
Robert Blake, taking part in an expedition into the
Mediterranean. This experience made him a leading advocate of establishing a British naval base in the region, an ambition realised with the acquisition of
English Tangier in 1661. In February 1657, he was one of the so-called "New Cromwellians" who supported the
Humble Petition and Advice, inviting Cromwell to declare himself king and advocating the re-establishment of a national church. The measure was opposed by army radicals including
Charles Fleetwood and
John Lambert and ultimately rejected. In June 1658 he commanded the naval squadron that blockaded
Dunkirk and when Cromwell died in September, Montagu remained loyal to his son and appointed successor
Richard Cromwell. During his brief and disastrous rule as
Lord Protector, Montagu remained at sea and in early 1659 was sent to mediate between Sweden and Denmark; however, he was suspected of secret communication with the exiled
Charles II and the republicans
Algernon Sidney and
Sir Robert Honywood were sent to monitor his activity. He was recalled and investigated by the newly installed
Rump Parliament; although no evidence was found, he was dismissed from command. ==Restoration==