(1650–1652) The leaders of the English Commonwealth felt threatened by the Scots reassembling an army. They pressed Thomas Fairfax, lord general of the New Model Army, to launch a pre-emptive attack. Fairfax accepted the commission to lead the army north to defend against the possibility of a Scottish invasion, but was unwilling to strike the first blow against his former allies, believing England and Scotland were still bound by the Solemn League and Covenant. When a formal order to attack came on 20 June 1650, Fairfax resigned his commission. A parliamentary committee which included Cromwell, his close friend, attempted to dissuade him, pleading with him over the course of a whole night to change his mind, but Fairfax remained resolute, and retired from public life. Cromwell succeeded to his office as lord general, becoming commander-in-chief of the New Model Army. He received his commission on 28 June, and set out for Scotland the same day, crossing the Tweed at the head of 16,000 men on 22 July. Once the Treaty of Breda had been signed, the Scottish Parliament started levying men to form a new army, under the command of the experienced general David Leslie. Their aim was to increase their forces to more than 36,000 men, but that number was never achieved; by the time Cromwell entered Scotland, Leslie had fewer than 10,000 infantry and 3,000 cavalry, although these numbers fluctuated during the course of the campaign. The government instituted a commission to purge the army of anyone suspected of having supported the Engagement, as well as men considered sinful or undesirable. This was opposed, unsuccessfully, by much of the Scottish nobility and most of the experienced military leaders, including Leslie. The purge removed many experienced men and officers, and the bulk of the army was composed of raw recruits with little training or experience. Leslie prepared a defensive line of earthworks between Edinburgh and Leith, and employed a scorched earth policy between this line and the English border. He then allowed Cromwell to advance unopposed. Lack of supplies, and the hostility of the local people towards the English invaders forced Cromwell to rely on a seaborne supply chain, and he captured the ports of Dunbar and Musselburgh to facilitate this. Operations were hampered by persistent bad weather. These adverse conditions caused a shortage of food and much sickness in the English army, substantially reducing its strength. Cromwell attempted