Gustavus Adolphus
Military commander

Gustavus Adolphus inherited three wars from his father when he ascended the throne: against Denmark–Norway, which had attacked Sweden earlier in 1611; against Russia, due to Sweden having tried to take advantage of the Russian Time of Troubles; and against Poland-Lithuania, due to King Charles's having deposed King Sigismund III, his nephew, as King of Sweden.

The war against Denmark–Norway (Kalmar War), during which Gustavus fought in minor military actions, — the victorious for Sweden Storming of Kristianopel and the unsuccessful Battle of Vittsjö, — was concluded in 1613 with a peace that did not cost Sweden any territory except for Älvsborg Castle, which Sweden had to pay to get back, but it was forced to pay a heavy indemnity to Denmark–Norway (Treaty of Knäred). During this war, Gustavus Adolphus let his soldiers plunder towns and villages, and as he met little resistance from Danish forces in Scania, they pillaged and devastated twenty-four Scanian parishes. His memory in Scania has been negative because of that fear. The largest destroyed settlement was the Town Væ, which two years later was replaced by Danish–Norwegian King Christian IV as the nearby Christiansted (after the Swedification process, spelled Kristianstad), the last Scanian town to be founded by a Danish king.

The war against Russia (Ingrian War) marked Gustavus' involvement in the successful Siege of Gdov and the failed Siege of Pskov and ended in 1617 with the Treaty of Stolbovo, which excluded Russia from the Baltic Sea. The final inherited war, the war against Poland, ended in 1629 with the Truce of Altmark, which transferred the large province of Livonia to Sweden and freed the Swedish forces for the subsequent intervention in the Thirty Years' War in Germany, where Swedish forces had already established a bridgehead in 1628.

The electorate of Brandenburg was especially torn apart by a quarrel between the Protestant and Catholic parties. The Brandenburg minister and diplomat baron Samuel von Winterfeld influenced Gustavus Adolphus to support and protect the Protestant side in Germany. When Gustavus Adolphus began his push into northern Germany in June–July 1630, he had just 4,000 troops. He was soon able to consolidate the Protestant position in the north, however, using reinforcements from Sweden and money supplied by France at the Treaty of Bärwalde.

After Swedish plundering in Brandenburg (1631) endangered the system of retrieving war contributions from occupied territories, "marauding and plundering" by Swedish soldiers was prohibited. Meanwhile, a Catholic army under Johann Tserclaes, Count of Tilly was laying waste to Saxony. Gustavus Adolphus met Tilly's army and won a decisive victory at the First Battle of Breitenfeld in September 1631. He then marched across Germany, establishing his winter quarters near the Rhine, making plans for the invasion of the rest of the Holy Roman Empire.

In March 1632, Gustavus Adolphus invaded Bavaria, an ally of the Emperor. He forced the withdrawal of his Catholic opponents at the Battle of Rain, marking the high point of the campaign. In the summer of that year, he sought a political solution that would preserve the existing structure of states in Germany, while guaranteeing the security of its Protestants. But achieving these objectives depended on his continued success on the battlefield.

Some other military actions in the Thirty Years' War with Gustavus at the head were: the victorious battles of Frankfurt an der Oder and Werben, the botched Siege of Nuremberg, the Battle of Fürth, and the unfavourable Battle of the Alte Veste.

Gustavus Adolphus is reported to have entered battle without wearing any armor, proclaiming, "The Lord God is my protector!" However, it is more likely that he simply wore a padded cuirass rather than going into battle wearing no battle protection whatsoever. In 1627, near Dirschau in Prussia, a Polish soldier shot him in the muscles above his shoulders. He survived, but the doctors could not remove the bullet, so from that point on, he could not wear iron armor and two fingers of his right hand were paralyzed. The plate cuirass normally worn by important officers at that time was replaced by a buff coat made of moose hide, which would have serious consequences later.
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