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.