France’s Opposition to the Holy Roman Empire

France, though Roman Catholic, was a rival of the Holy Roman Empire and Spain. Cardinal Richelieu, the chief minister of King Louis XIII of France, considered the Habsburgs too powerful because they held a number of territories on France’s eastern border, including portions of the Netherlands. Richelieu had already begun intervening indirectly in the war in January 1631, when the French diplomat Hercule de Charnacé signed the Treaty of Bärwalde with Gustavus Adolphus, by which France agreed to support the Swedes with 1,000,000 livres each year in return for a Swedish promise to maintain an army in Germany against the Habsburgs. The treaty also stipulated that Sweden would not conclude a peace with the Holy Roman Emperor without first receiving France’s approval.

France Enters the War

After the Swedish rout at Nördlingen in September 1634 and the Peace of Prague in 1635, in which the Protestant German princes sued for peace with the German emperor, Sweden’s ability to continue the war alone appeared doubtful, and Richelieu made the decision to enter into direct war against the Habsburgs. France declared war on Spain in May 1635, and on the Holy Roman Empire in August 1636, opening offensives against the Habsburgs in Germany and the Low Countries. France aligned its strategy with the allied Swedes in Wismar (1636) and Hamburg (1638).

Early French military efforts were met with disaster, and the Spanish counter-attacked, invading French territory. The imperial general Johann von Werth and Spanish commander Cardinal-Infante Ferdinand ravaged the French provinces of Champagne, Burgundy, and Picardy, and even threatened Paris in 1636. Then, the tide began to turn for the French. The Spanish army was repulsed by Bernhard of Saxe-Weimar. Bernhard’s victory in the Battle of Compiègne pushed the Habsburg armies back towards the borders of France. Then widespread fighting ensued until 1640, with neither side gaining an advantage.

However, the war reached a climax and the tide of the war turned clearly toward France and against Spain in 1640, starting with the siege and capture of the fort at Arras. The French conquered Arras from the Spanish following a siege that lasted from June 16 to August 9, 1640. When Arras fell, the way was opened for the French to take all of Flanders. The ensuing French campaign against the Spanish forces in Flanders culminated with a decisive French victory at Rocroi in May 1643.

Continued Swedish War Efforts

After the Peace of Prague, the Swedes reorganized the Royal Army under Johan Banér and created a new one, the Army of the Weser, under the command of Alexander Leslie. The two army groups moved south in the spring of 1636, re-establishing alliances on the way, including a revitalized one with Wilhelm of Hesse-Kassel. The two Swedish armies combined and confronted the imperialists at the Battle of Wittstock. Despite the odds being stacked against them, the Swedish army won. This success largely reversed many of the effects of their defeat at Nördlingen, albeit not without creating some tensions between Banér and Leslie.

After the battle of Wittstock, the Swedish army regained the initiative in the German campaign. In the Second Battle of Breitenfeld, in 1642, outside Leipzig, the Swedish Field Marshal Lennart Torstenson defeated an army of the Holy Roman Empire led by Archduke Leopold Wilhelm of Austria and his deputy, Prince-General Ottavio Piccolomini, Duke of Amalfi. The imperial army suffered 20,000 casualties. In addition, the Swedish army took 5,000 prisoners and seized forty-six guns, at a cost to themselves of 4,000 killed or wounded. The battle enabled Sweden to occupy Saxony and impressed on Ferdinand III the need to include Sweden, and not only France, in any peace negotiations.

Final Battles

Over the next four years, fighting continued, but all sides began to prepare for ending the war. In 1648, the Swedes (commanded by Marshal Carl Gustaf Wrangel) and the French (led by Turenne and Condé) defeated the imperial army at the Battle of Zusmarshausen, and the Spanish at Lens. However, an imperial army led by Octavio Piccolomini managed to check the Franco-Swedish army in Bavaria, though their position remained fragile. The Battle of Prague in 1648 became the last action of the Thirty Years’ War. The general Hans Christoff von Königsmarck, commanding Sweden’s flying column, entered the city and captured Prague Castle (where the event that triggered the war—the Defenestration of Prague—had taken place thirty years before). There, they captured many valuable treasures, including the Codex Gigas, which contains the Vulgate Bible as well as many historical documents all written in Latin, and is still today preserved in Stockholm as the largest extant medieval manuscript in the world. However, they failed to conquer the right-bank part of Prague and the old city, which resisted until the end of the war. These results left only the imperial territories of Austria safely in Habsburg hands.

The Swedish siege of Prague in 1648

In 1648, the Swedish army entered Prague and captured Prague Castle, where the catalyst of the war, the Defenestration of Prague, had taken place thirty years before.