Until the advent of railways in the 19th century, water was the primary means of bulk transportation, and campaigns focused on control of rivers and ports. 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; The French army of 27,000 that invaded Flanders in May 1635 was reduced by desertion and disease to under 17,000 by early July.