Three decades of campaigning, battling and local skirmishes, now called the Thirty Years’ War, started in Prague, from a clash between the Czech estates and the Emperor and his councillors representing the Emperor at the Prague Castle. The rather comic defenestration, when the noblemen Slavata and von Martinicz, along with a scribe, landed on a heap of much and rubbish beneath the windows of their office, started the longest and up until then the bloodiest conflict in history. Although two years later the Czech estates lost their cause at the Battle of the White Mountain, the conflict spread beyond the borders of Bohemia to Germany, where the local Catholic and Protestant princes fought against each other for the next ten years. The war was more of an internal German matter during this period, as a violent attempt to decide the future of political and religious freedoms, even though the army of the Danish king made a brief appearance on the German battlefield.
In 1631, Swedish forces landed on the German Baltic shores to help their fellow Protestants. Thus, the internal German conflict became a European war. Gradually, it involved the Spanish Habsburgs on the Imperial side, while the Catholic France joined the Protestant Swedes. The Dutch hoped that a defeat of the Imperial side would weaken the Habsburg position in Spain. The exploding conflict gradually involved more and more combatants. Numerous princes changed sides during the war from the Swedish to the Imperial side, and regardless of that, armies on both sides employed mercenaries as well as conscripts from a number of nations, Germans, Englishmen, Italians, Swedes, Czechs, Jews, Croatians, or Turks. This made the conflict “global” in terms of the period.
Both warring parties were quite exhausted at the beginning of 1645. Financial issues existed in armies propped up by mercenaries from the very beginning, the pay was getting late and the hungry troops regularly looted the occupied territories, regardless of whether the locals sympathised with their own side or the enemy. Luckily, each of the warring parties could afford only one army, moving about Europe. The longer the troops stayed somewhere, the more desolate and inhabitable the land was. This way, much of Bohemia and Germany, part of Poland, Denmark and some Swedish territories were devastated. No matter how bloody the battles were, more people died of famine, diseases and during looting and the struggle to feed the tens of thousands strong armies. A definitive peace was contemplated and negotiated from 1635; however, both warring parties simultaneously hoped to wage and win the decisive battle to gain the upper hand. The campaign, of which the Battle of Jankov was a part, was such one attempt.
In autumn, 1644 the commander of the Swedish forces Marshal Lennart Torstensson decided to launch a quick attack against Vienna and definitely defeat the Catholic Emperor Ferdinand III. He set off from Leipzig, where the army wintered, crossed the border mountains near the West Bohemian Kadan, and swung south. The Emperor decided that the campaign must be stopped at all costs. The Imperial army came to Bohemia from Poland and Hungary to meet the Swedes and prevent them from crossing the Vltava River or engage them in a decisive battle if necessary. Both armies manoeuvred in the part of the country between the towns of Klatovy, Strakonice and Tabor. When the Swedes crossed the frozen Vltava, nothing stood between them and the road to Moravia and then on to Vienna. The decisive battle was inevitable.