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 their ability to conduct operations. Sickness killed far more soldiers than combat; the French army that invaded Flanders in May 1635 was reduced by desertion and disease from 27,000 to under 17,000 by early July. Background 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. Most of the Holy Roman Empire remained neutral, viewing it as an inheritance dispute, and the revolt was quickly suppressed. However, when Frederick refused to admit defeat, Imperial forces invaded the Palatinate and forced him into exile; removal of a hereditary prince changed the nature and extent of the war. Combined with a renewed Counter-Reformation, it presented a direct threat both to Protestant states within the Empire and external powers who held Imperial territories. They included the Dutch Prince of Orange, hereditary ruler of Nassau-Dillenburg, and Christian IV of Denmark, who was also Duke of Holstein-Gottorp. This presented Richelieu with additional opportunities to weaken his Habsburg opponents in Spain and the Empire, while avoiding direct conflict. As a result, Catholic France supported the Protestant Dutch Republic in its war with Spain, as well as funding first Danish, then Swedish intervention in the Empire. In 1630, Gustavus Adolphus of Sweden invaded Pomerania; while done partly to support his Protestant co-religionists, he also sought control of the Baltic trade, which provided much of Sweden's income. Swedish intervention continued after his death at Lützen in 1632, but caused tensions with Saxony and Brandenburg-Prussia, whose lands were devastated by the plague and famine that accompanied the war. A significant Imperial-Spanish victory at Nördlingen in September 1634 forced the Swedes to abandon southern Germany, while most of their German allies used the opportunity to make peace with Ferdinand II at Prague in April 1635. The other major European conflict of the period was the Eighty Years' War (1568-1648) between Spain and the Dutch Republic, suspended in 1609 by the Twelve Years' Truce. The Spanish strongly objected to its commercial provisions and when Philip IV became king in 1621, he resumed the war.