Battle of Breitenfeld (1631)  The Battle of Breitenfeld (; ) or First Battle of Breitenfeld (in older texts sometimes known as Battle of Leipzig), was fought at a crossroads near Breitenfeld approximately 8 km north-west of the walled city of Leipzig on September 17 (Gregorian calendar), or September 7 (Julian calendar, in wide use at the time), 1631. It was the Protestants' first major victory of the Thirty Years War. The victory confirmed Sweden's Gustavus Adolphus of the House of Vasa as a great tactical leader and induced many Protestant German states to ally with Sweden against the German Catholic League, led by Maximilian I, Elector of Bavaria, and the Holy Roman Emperor Ferdinand II. The Swedish phase of the Thirty Years War began when Gustavus Adolphus and his force of 13,000 landed at Peenemünde in 1630. Initially, Sweden's entrance into the war was considered a minor annoyance to the Catholic League and its allies; his only battles to this point had been inconclusive ones, or fought against generals of modest military ability. Consequently, the Imperial Commander of the German Catholic League, Tilly, did not immediately respond to the arrival of the Swedes, being engaged in northern Italy. However, the effective end of the Mantuan War in 1631 ensured that the large Imperial army previously tied up there was now free to move into the German states. When the Protestant princes showed little interest in attaching themselves to the Swedish cause, Gustavus opted for “rough wooing.” His troops moved south into Brandenburg, taking and sacking the towns of Küstrin and Frankfurt an der Oder. It was too late and too far to save one of Gustav's “occupied” allies, Magdeburg, from a horrific sack by Imperial troops, beginning on May 20, in which a major portion of the population was murdered and the city burned. The Swedes turned the sack of Magdeburg to good use: broadsides and pamphlets distributed throughout Europe ensured that prince and pauper alike understood how the Emperor, or at least his troops, treated his Protestant subjects. Over the next few months, Gustavus consolidated his bridgehead and expanded across northern Germany, attracting support from German princes and building his army from mercenary forces along the way. By the time he reached the Saxon border, his force had grown to over 23,000 men.