 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.The Thirty Years War began in 1618 when the Protestant-dominated Bohemian Estates offered the Crown of Bohemia to Frederick of the Palatinate, rather than the conservative Catholic, Emperor Ferdinand II.