If Maximilian of Bavaria desired the title of elector as his reward for supporting Ferdinand, Spain (for its part) required imperial support for its war against the Dutch. When repeated requests for a direct invasion by Wallenstein’s army remained unanswered (largely due to Bavarian opposition), Spain began to think of creating a Baltic navy, with imperial assistance, which would cleanse the inland sea of Dutch shipping and thus administer a body blow to the republic’s economy. But the plan aborted, for the imperial army failed in 1628 to conquer the port of Stralsund, selected as the base for the new fleet. Now, with Denmark defeated, Madrid again pleaded for the loan of an imperial army, and this time the request was granted. In the end, however, the troops did not march to the Netherlands: instead, they went to Italy.

The death of the last native ruler of the strategic states of Mantua and Montferrat in December 1627 created dangers in Italy that the Spaniards were unable to ignore and temptations that they were unable to resist. Hoping to forestall intervention by others, Spanish forces from Lombardy launched an invasion, but the garrisons of Mantua and Montferrat declared for the late duke’s relative, the French-born duke of Nevers. Nevers lacked the resources to withstand the forces of Spain alone, and he appealed to France for support. Louis XIII (1610–43) and Cardinal Richelieu (chief minister 1624–42) were, however, engaged in a desperate war against their Calvinist subjects; only when the rebels had been defeated, early in 1629, was it possible for the king and his chief minister to cross the Mount Cenis Pass and enter Italy. It was to meet this threat that the emperor was asked by Philip IV of Spain (1621–65) to send his troops to Italy rather than to the Netherlands. When Louis XIII launched a second invasion in 1630, some 50,000 imperial troops were brought south to oppose them, reducing the war for Mantua to a stalemate but delivering the Dutch Republic from immediate danger and weakening the emperor’s hold on Germany.

Gustav II Adolf of Sweden (1611–32) had spent most of the 1620s at war with Poland, seeking to acquire territory on the southern shore of the Baltic. By the Truce of Altmark (September 26, 1629), with the aid of French and British mediators, Poland made numerous concessions in return for a six-year truce. Gustav lost no time in redeploying his forces: on July 6, 1630, he led a Swedish expeditionary force ashore near Stralsund with the declared intention of saving the “liberties of the empire” and preserving the security of the Baltic.

Despite the defeat of the German Protestants and their allies, Sweden’s position was far more favourable than that of Denmark five years earlier. Instead of the two armies that had faced Christian IV, Gustav was opposed by only one, for in the summer of 1630 the emperor’s Catholic allies in Germany—led by Maximilian of Bavaria—demanded the dismissal of Wallenstein and the drastic reduction of his expensive army. It was an ultimatum that Ferdinand, with the bulk of his forces tied down in the war of Mantua, could not ignore, even though he thereby lost the services of the one man who might conceivably have retained all the imperial gains of the previous decade and united Germany under a strong monarchy.

The emperor and his German allies, nevertheless, did remain united over the Edict of Restitution: there were to be no concessions in matters of religion and no restoration of forfeited lands. As a result, the German Protestants were driven reluctantly into the arms of Sweden, whose army was increased with the aid of subsidies secured from France and the Dutch. In September 1631 Gustav at last felt strong enough to challenge the emperor’s forces in battle: at Breitenfeld, just outside Leipzig in Saxony, he was totally victorious. The main Catholic field army was destroyed, and the Swedish Protestant host overran most of central Germany and Bohemia in the winter of 1631–32. The next summer they occupied Bavaria. Although Gustav died in battle at Lützen on November 16, 1632, his forces were again victorious and his cause was directed with equal skill by his chief adviser, Axel Oxenstierna. In the east, Sweden managed to engineer a Russian invasion of Poland in the autumn of 1632 that tied down the forces of both powers for almost two years. Meanwhile, in Germany, Oxenstierna crafted a military alliance that transferred much of the cost of the war onto the shoulders of the German Protestant states (the Heilbronn League, April 23, 1633). Swedish ascendancy, however, was destroyed in 1634 when Russia made peace with Poland (at Polyanov, June 4) and Spain sent a large army across the Alps from Lombardy to join the imperial forces at the Battle of Nördlingen (September 6). This time the Swedes were decisively beaten and were obliged to withdraw their forces in haste from most of southern Germany.

Yet Sweden, under Oxenstierna’s skillful direction, fought on. Certainly its motives included a desire to defend the Protestant cause in Germany and to restore deposed princes to their thrones; but more important by far was the fear that, if the German Protestants were finally defeated, the imperialists would turn the Baltic into a Habsburg lake and might perhaps invade Sweden. The Stockholm government therefore desired a settlement that would atomize the empire into a jumble of independent, weak states incapable of threatening the security of Sweden or its hold on the Baltic. Furthermore, to guarantee this fragmentation, Oxenstierna desired the transfer to his country of sovereignty over certain strategic areas of the empire—particularly the duchy of Pomerania on the Baltic coast and the electorate of Mainz on the Rhine.

These, however, were not at all the goals of Sweden’s German allies. They aimed rather at the restoration of the prewar situation—in which there had been no place for Sweden—and it soon became clear that they were prepared to make a separate settlement with the emperor in order to achieve it. No sooner was Gustav dead than the elector of Saxony, as “foremost Lutheran prince of the Empire,” put out peace feelers toward Vienna. At first John George (1611–56) was adamant about the need to abolish the Edict of Restitution and to secure a full amnesty for all as preconditions for a settlement; but the imperial victory at Nördlingen made him less demanding. The insistence on an amnesty for Frederick V was dropped, and it was accepted that the edict would be applied in all areas recovered by Catholic forces before November 1627 (roughly speaking, this affected all lands south of the Elbe, but not the Lutheran heartland of Saxony and Brandenburg). The elector might have been required to make even more concessions but for the fact that, over the winter of 1634–35, French troops began to mass along the borders of Germany. As the papal nuncio in Vienna observed: “If the French intervene in Germany, the emperor will be forced to conclude peace with Saxony on whatever terms he can.” So the Peace of Prague was signed between the emperor and the Saxons on May 30, 1635, and within a year most other German Lutherans also changed their allegiance from Stockholm to Vienna.