[Sidenote: 3. Swedish Intervention: Gustavus Adolphus]
It was likewise at the very time when Cardinal Richelieu had crushed out all insurrection, whether Huguenot or noble, in France and was seeking some effective means of prolonging the war in the Germanies to the end that the rival Habsburgs might be irretrievably weakened and humiliated. He entered into definite alliance with Gustavus Adolphus and provided him arms and money, for the time asking only that the Protestant champion accord the liberty of Catholic worship in conquered districts.
[Sidenote: French Aid]
Gustavus Adolphus landed in Pomerania in 1630 and proceeded to occupy the chief northern fortresses and to treat for alliances with the influential Protestant electors of Brandenburg and Saxony. While Gustavus tarried at Potsdam, in protracted negotiation with the elector of Brandenburg, Tilly and the imperialists succeeded, after a long siege, in capturing the Lutheran stronghold of Magdeburg (May, 1631). The fall of the city was attended by a mad massacre of the garrison, and of armed and unarmed citizens, in streets, houses, and churches; at least 20,000 perished; wholesale plundering and a general conflagration completed the havoc. The sack of Magdeburg evoked the greatest indignation from the Lutherans. Gustavus Adolphus, now joined by the electors of Brandenburg and Saxony and by many other Protestant princes of northern Germany, advanced into Saxony, where, in September, 1631, he avenged the destruction of Magdeburg by defeating decisively the smaller army of Tilly on the Breitenfeld, near Leipzig. Then Gustavus turned southwestward, making for the Rhine valley, with the idea of forming a union with the Calvinist princes. Only the prompt protest of his powerful ally, Richelieu, prevented the rich archbishoprics of Cologne, Trier, and Mainz from passing immediately under Swedish control. Next Gustavus Adolphus turned east and invaded Bavaria. Tilly, who had reassembled his forces, failed to check the invasion and lost his life in a battle on the Lech (April, 1632). The victorious Swedish king now made ready to carry the war into the hereditary dominions of the Austrian Habsburgs. As a last resort to check the invader, the emperor recalled Wallenstein with full power over his freelance army. About the same time the emperor concluded a close alliance with his kinsman, the ambitious Philip IV of Spain.
The memorable contest between the two great generals—Gustavus Adolphus and Wallenstein—was brought to a tragic close in the late autumn of the same year on the fateful field of Lützen. Wallenstein was defeated, but Gustavus was killed. Although the Swedes continued the struggle, they were comparatively few in numbers and possessed no such general as their fallen king. On the other side, Wallenstein's loyalty could not be depended upon; rumors reached the ear of the emperor that his foremost general was negotiating with the Protestants to make peace on his own terms; and Wallenstein was assassinated in his camp by fanatical imperialists (February, 1634). The tragic removal of both Wallenstein and Gustavus Adolphus, the economic exhaustion of the whole empire, and the national desire on the part of many Protestant princes, as well as on the part of the Catholic emperor, to rid the Germanies of foreign soldiers and foreign influence—all these developments seemed to point to the possibility of concluding the third, or Swedish, period of the war, not perhaps as advantageously for the imperialist cause as had ended the Bohemian revolt or the Danish intervention, but at any rate in a spirit of reasonable compromise. In fact, in May, 1635, a treaty was signed at Prague between the emperor and such princes as were then willing to lay down their arms, whereby all the military forces in the empire were henceforth to be under the direct control of the emperor (with the exception of a contingent under the special command of the Lutheran elector of Saxony); all princely leagues within the empire were to be dissolved; mutual restoration of captured territory was to be made; and, as to the fundamental question of the ownership of ecclesiastical lands, it was settled that any such lands actually held in the year 1627, whether acquired before or after the religious peace of Augsburg of 1555, should continue so to be held for forty years or until in each case an amicable arrangement could be reached.
What wrecked the peace of Prague was not so much the disinclination of the Protestant princes of Germany to accept its terms as the policy of Cardinal Richelieu of France. Richelieu was convinced more than ever that French greatness depended upon Habsburg defeat; he would not suffer the princes to make peace with the emperor until the latter was soundly trounced and all Germany devastated; instead of supplying the Swedes and the German Protestants with assistance from behind the scenes, he now would come boldly upon the stage and engage the emperor in open combat.
[Sidenote: 4. French Intervention. Richelieu's Policy in the Germanies]
The final, or French, period of the Thirty Years' War lasted from 1635 to 1648—almost as long as the other three periods put together. Richelieu entered the war not only to humble the Austrian Habsburgs and, if possible, to wrest the valuable Rhenish province of Alsace from the Holy Roman Empire, but also to strike telling blows at the Continental supremacy of the Spanish Habsburgs, who, since 1632, had been actively helping their German kinsmen. The Spanish king, it will be remembered, still held the Belgian Netherlands, on the northern frontier of France, and Franche Comté on the east, while oft-contested Milan in northern Italy was a Spanish dependency. France was almost surrounded by Spanish possessions, and Richelieu naturally declared war against Spain as against the emperor. The wily French cardinal could count upon the Swedes and many of the German Protestants to keep the Austrian Habsburgs busily engaged and upon the assistance of the Dutch in humbling the Spaniard, for Spain had not yet formally recognized the independence of the Dutch Netherlands. Inasmuch as England was chiefly concerned with troublesome internal affairs, the enemies of France could hardly expect aid from across the Channel.
[Sidenote: Condé and Turenne]
At first, the French suffered a series of military reverses, due in large part to unpreparedness, incompetent commanders, and ill- disciplined troops. At one time it looked as if the Spaniards might capture Paris. But with unflagging zeal and patriotic devotion, Richelieu pressed on the war. He raised armies, drilled them, and dispatched them into the Netherlands, into Alsace, into Franche Comté, into northern Italy, and into Roussillon. He stirred up the Portuguese to revolt and recover their independence (1640). And Mazarin, who succeeded him in 1642, preserved his foreign policy intact. Young and brilliant generals now appeared at the head of the French forces, among whom were the dashing Prince of Condé (1621-1686), and the master strategist Turenne (1611-1675), the greatest soldier of his day. The former's victory of Rocroi (1643) dated the commencement of the supremacy of France in war, a supremacy which was retained for a century.