Yet the Polish war was not thrown away. By the treaty of Stuhmsdorf, Sweden gained the whole of Livonia, and some places in Prussia, and by the training both of himself and of his army in the four Polish campaigns, he had unconsciously made Sweden one of the most formidable military powers of the day. [Sidenote: =Negotiations between England and Gustavus in 1624.=] While the Thirty Years’ War was in progress, the eyes both of Catholics and Protestants in Germany had often been turned towards Gustavus in fear and in hope. He himself looked forward with eagerness to the day when his assistance might be necessary, for he longed to cross swords with Tilly and the imperial generals, but it was eagerness tempered with prudence. He would enter into the war at his own time, and on his own terms, or not at all. In 1624 he was asked by England to formulate those terms, and he laid down three conditions as indispensable, that he should have the sole military management of the war, that England should provide the money for 17,000 men, and pay the subsidies for five months in advance, that he should be protected from attack from Denmark, while at war in Germany, and have two ports made over to him to secure his communications. Unlike Christian of Denmark, he would not be content with fair promises, but insisted on performance before he would move. The terms were too onerous for acceptance at that time, but the fate of Christian proved their wisdom and necessity. The defeat of the Danes, and the establishment of Wallenstein on the Baltic coast, brought the danger nearer home. What chance was there for Sweden to obtain supremacy over the Baltic with Mecklenberg and Pomerania in the hands of the imperial admiral? Clearly she would have to fight for her independence, let alone her religion, if Wallenstein was suffered to make himself duke of Mecklenberg. Gustavus recognised the necessity at once. [Sidenote: =Alliance between Sweden and Denmark, 1628.=] In April 1628 he made an alliance with his old enemy Christian IV. of Denmark, by which all foreign ships, except those of the Dutch were excluded from the Baltic. In the summer of the same year, he sent 2000 men under Alexander Leslie to defend Stralsund against Wallenstein. [Sidenote: =Landing of Gustavus in Germany, 1630.=]In September 1629 he put an end to the Polish war by the treaty of Stuhmsdorf, and on the 24th of June 1630 he landed on the island of Usedom, at the head of an army of 13,000 men, which was raised to 40,000 before the end of the year. [Sidenote: =Measures of Gustavus.=]