The former Swedish ally Saxony defected in the
Peace of Prague 1635 to Emperor
Ferdinand II. Swedish victory at
Wittstock in autumn 1636 led to the first brief Swedish incursion into Saxony but was overturned by an Imperial counteroffensive in 1637. The main Imperial army led by
Matthias Gallas had returned from
Burgundy and repulsed the Swedes back to
Pomerania. Over most of the year 1638, the Swedes were enclosed in a few coastal fortresses on the
Baltic Sea. However, while the Swedes under Banér were supplied and reinforced by the sea, the Imperials received little support by their allies
Brandenburg and Saxony and were unable to consistently supply their army in the already war-ravaged region. After Gallas was pushed back to
Mecklenburg by Banér in autumn and was denied winter quarters by the neighbouring
Lower Saxon Circle, he retreated with his shrinking army to
Bohemia. In January 1639, Banér seized the opportunity of Gallas' retreat and crossed the Elbe with the Swedish army. He went through
Lüneburg that chose to stay neutral but provided him with supplies. To keep his army intact and bring the war into the lands of the Emperor and his allies, Banér traversed the devastated area and attacked Saxony with 18,000 men. After he destroyed an Imperial detachment under Hans Wolf von Salis at
Elsterberg in March, he faced unexpected little resistance, taking
Zwickau and Chemnitz in quick succession. Only when he attacked
Freiberg, the silver mine of the Saxon Elector, the garrison vigorously resisted. Banér lost 500 men in an unsuccessful assault. When the Saxon army under Marazzino approached Freiberg with 5,000 men, Banér abandoned the siege and fell back via Chemnitz to join forces with
Lennart Torstensson. The Saxon commander underestimated the Swedish strength and pursued Banér to Chemnitz. However, Banér had reunited his force with Torstensson at
Zeitz on 12 April, boosting his overall strength to 20,000 men. He decided to attack the Saxons, now counting 8,000 men, before they could be reinforced by Imperials under
Melchior von Hatzfeldt who were approaching from
Thuringia. ==Battle==