Leslie prepared a defensive line of earthworks between Edinburgh and Leith, and employed a scorched earth policy between that line and the Scottish 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 in order to facilitate this. Operations were hampered by persistent bad weather, and the adverse conditions and a shortage of food caused much sickness in the English army, substantially reducing its strength.