Denmark was the foe upon whom Gustavus was called to whet his virgin steel. Taking advantage of the confusion caused by the minority of the new king, Christian IV. had seized upon Elfsborg and Calmar early in 1611. Directly Gustavus had been pronounced of age he marched to recover the fortresses, and learned his first lesson in the art of war in a year of frontier hostilities, which were ended through the mediation of James I. by the peace of Knarod in January 1613. By this treaty Calmar was at once restored to the Swedes, and Elfsborg covenanted to be restored on the payment of a million dollars, which were duly raised and paid in two years. Relieved from all present anxiety from the side of Denmark, Gustavus at once turned his attention to the growing power of Russia, now gathering itself together under Michael Romanoff. [Sidenote: =War with Russia, 1614–1617.=] In 1614 he invaded Ingria, and spent three years in desultory fighting, in which he was uniformly victorious in battle, and slowly occupied the country. Again England, who had trade relations with Russia, offered her mediation, and by the treaty of Stolbova, signed in February 1617, Sweden obtained from Russia the cession of Ingria and Carelia, thus gaining a continuous coast line on the Baltic from Calmar to Riga, and shutting Russia from the sea altogether. ‘The enemy,’ said Gustavus triumphantly, ‘cannot launch a boat upon the Baltic without our permission.’ [Sidenote: =War with Poland, 1617–1629.=] Hardly was the peace of Stolbova signed, than an invasion of Swedish Livonia by Sigismund of Poland forced Gustavus to enter upon his third war. Poland was a much more difficult nut to crack than Russia had been, for behind Sigismund lay the forces of the Counter-Reformation, but from various circumstances neither side could press the war with vigour. Two armistices (from 1618–1621 and from 1622–1625) interrupted its lethargic course, and enabled Sweden to recruit her failing energies, and her king to perfect the improvements in military tactics for which he is famous. In 1625 he resumed the war in earnest, and crossing the Dwina overran and occupied Courland, pushing the Polish generals back into Lithuania. But neither Riga nor any of the Courland towns gave him what he most wanted, a place of first-rate importance, which he might make the centre of his operations; so in the next year he directed his attack on Dantzig, although it involved the violation of the neutrality of his brother-in-law, George William of Brandenburg. Dantzig was a town strongly fortified on the land side. The Swedish fleet was too weak to enforce the attempted blockade by sea. Hence, like Stralsund and La Rochelle, until cut off from the sea it was impregnable. For four weary years Gustavus attempted unsuccessfully to reduce it. Eventually in 1629, when the affairs in Germany rendered it essential for him to have his hands free, he consented to make peace without gaining the desired end.