[2] 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.

[5] 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.

[7] 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.

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.

In February 1660, George Monck, military commander in Scotland, marched into London and declared his support for the Rump against the Republican faction led by John Lambert.

[11] This placed him in a powerful position during negotiations for the Restoration; when Parliament resolved to proclaim Charles king and invited him to return to England, Montagu commanded the fleet that brought him from the Dutch Republic on 24 May.

His reputation suffered another serious blow when he failed to prevent his sailors from plundering a number of Dutch VOC prize ships, loaded with precious spices from the East Indies, which he had brought in.

By long-standing custom the sailors could take any goods they found between the decks, but they were strictly forbidden to "break the bulk" i.e. ransack the ship's hold; yet this is just what Sandwich, an easy-going man with a notoriously poor understanding of money matters, permitted.

When this became widely known, the rumour spread that Sandwich had unlawfully helped himself to a fortune (in fact he seems to have taken less than he was entitled to), and the public, who were still enduring the horrors of the Great Plague of London, reacted with such unexpected fury that a minor mishap became a national affair: "the Prize Goods Scandal".

Like all Ambassadors of the era, he found the cost of running an embassy ruinous (he had never had a good head for business) and on his return to England in the autumn of 1668 one of his first actions was to borrow money from his cousin Samuel Pepys.

Of the existence of the Treaty's secret clauses, notably that by which Charles II pledged to convert to the Roman Catholic faith, Sandwich, like the general public, was quite unaware.

[21] In the same year he was appointed President of the Privy Council Committee on Foreign Plantations; he had always had a keen interest in international trade, despite his notorious inability to keep his own finances in order.

In 1668 Pepys was somewhat perturbed when his wife Elizabeth, during one of the violent quarrels which followed the discovery of his affair with her companion Deb Willet, told him that Sandwich had asked her to be his mistress.

[25] Since Pepys was in no doubt that she had refused, he decided to treat the matter as being closed, and friendly relations continued: Sandwich dined at their house for the first time a few months later.