The Battle of Nördlingen was part of the Thirty Years' War, fought from 1618 to 1648. The chief belligerents were the Catholic Habsburg dynasties consisting of an Austrian and Spanish branch and their allies on one side. (The Austrian archduke also held the title of Holy Roman Emperor. For this reason, the Austrian Habsburgs are frequently referred to as the Imperialists.) Opposed to them were the Protestant nations comprising the Dutch, Denmark, Sweden, various German principalities and later, Catholic France.
After the Protestant victory at the Battle of Lützen two years before, the Swedes failed to follow up due to the death of their king, Gustavus Adolphus. As a result, the Imperial forces began to regain the initiative.
In 1634 Protestant German and Swedish forces moved south and invaded Bavaria, threatening a major Habsburg ally. In response, the Austrian Habsburg commander, Ferdinand of Hungary (son of Ferdinand II, the Holy Roman Emperor) advanced west from Bohemia (today, the Czech Republic) threatening to cut across the supply lines of the Protestant armies. Consequently, the Protestant commanders quickly reversed course and headed north. They were aware that Spanish reinforcements under Ferdinand of Hungary's cousin, the Cardinal-Infante Ferdinand, were en route from their dominions in Northern Italy. The Spanish army had marched through the Stelvio Pass trying to open a new "Spanish Road", and take their Commander to his Governorship in the Spanish Low Countries.
The Protestant commanders decided they could not ignore the threat of a union between the two enemy forces and combined their two largest armies near Augsburg on 12 July, which included the Swabian-Alsatian Army under Gustav Horn and the so-called Franconian Army under Bernhard of Saxe-Weimar. Both armies were named after their main operation area and belonged to the Heilbronn Alliance (Sweden's German-Protestant allies under the directorate of the Swedish chancellor Axel Oxenstierna). These forces mostly consisted of German recruits. Among them were "the Blue brigade" and some Scottish allies ("the Green brigade") with a few national Swedish/Finnish regiments (mostly cavalry) and one national Swedish infantry brigade ("the Yellow brigade").
The Protestants proved unable to prevent the fall of Regensburg to Ferdinand of Hungary and desperately pursued him westwards in an effort to prevent the merger of the two Habsburg armies. On 16 August the Cardinal-Infante crossed the Danube at Donauwörth. Despite their best efforts the Protestant armies were still behind when Ferdinand of Hungary set down to besiege the town of Nördlingen in Swabia and await the Cardinal-Infante, who arrived before the city on 2 September - three days before the Protestants.