 Armies relied on foraging, while feeding the draught animals essential for transport and cavalry restricted campaigning in the winter.By the 1630s, the countryside had been devastated by years of constant warfare, which limited the size of armies, and the ability to conduct operations.Sickness killed far more soldiers than battle; France invaded Flanders with 27,000 men in May, which was reduced by desertion and disease to under 17,000 in early July.