Following a proposal by Admiral Edward Vernon, the concept of a Western Squadron was pioneered, which proved very successful. This marked a radical shift in British naval strategy, and led to British success at the Battles of Cape Finisterre. The following year, Sandwich took a commission as a colonel in the British Army as part of the response to the Jacobite rising and the prospect of a French invasion. In order to boost the relatively small British army, a number of units were raised by prominent figures, and Sandwich served in the regiment formed by Bedford. While serving in The Midlands, he fell seriously ill with fever and nearly died. After his recovery, he returned to his duties at the admiralty. He remained an army officer for the rest of his life, remaining on the half-pay list and eventually rising to the rank of general, even though he took no further active part in the army. In 1746 he was sent as a plenipotentiary to the Congress of Breda, and he continued to take part in the negotiations for peace until the Treaty of Aix-la-Chapelle was concluded in 1748. Sandwich was also made ambassador to the Dutch Republic during the talks. Using the resources of the British secret service, Sandwich was able to outmanoeuvre his French counterpart by intercepting the latter's secret correspondence. His service at Breda drew him to the attention of the influential Thomas Pelham-Holles, 1st Duke of Newcastle, who lobbied for him to be given high office when he returned home. It is possible that during his time at Breda, he played a role in the 1747 Dutch Revolution which brought William IV, Prince of Orange wider powers, something supported by Britain as they hoped the Prince would improve the Dutch Republic's military performance in the ongoing war in the Low Countries. However, there is no firm evidence of this. In February 1748 he became First Lord of the Admiralty, retaining this post until June 1751. By 1751 Newcastle, who had previously admired Sandwich for his forthright and hardline views, had increasingly begun to distrust him and his relationship with The Duke of Bedford whom Newcastle regarded as a rival. Newcastle engineered the dismissal of both of them, by sacking Sandwich. Bedford resigned in protest, as Newcastle had calculated, allowing him to replace them with men he considered more loyal personally to him. For the next few years Sandwich spent time at his country estate, largely avoiding politics, though he kept in close contact with both Bedford and Anson and Britain's participation in the Seven Years' War.