Gustavus Adolphus and his foreign policy is primarily associated with the Thirty Years War. When Sweden joined this war in 1630, her forces had not been tried out against what would have been considered major opposition. Up until his death at the Battle of Lutzen in 1632, Gustavus had an impressive record of success in the Thirty Years War.
However, Sweden had been involved in a number of Baltic wars before her involvement in the Thirty Years War.
The Kalmar War
This war was fought with Denmark between 1611 and 1613. It was a struggle with Christian IV for supremacy in the Baltic.
In May 1612, Sweden lost the fortress of Alvsborg. This
was a fortress that defended Sweden’s only access to the Atlantic and the
lucrative financial markets that lay beyond the Baltic. Christian could not
build on this success and he failed to conquer Sweden itself. He was seen as
being too aggressive by the Dutch and the Hanseatic League who feared he might
destabilise the whole area and the valuable trade associated with it. They
therefore sided with Sweden and this potentially powerful force persuaded
Christian to sue for peace. In January 1613, he signed the Treaty of Knared.
Sweden had to pay a large ransom for the return of Alvsborg (which she managed
to pay in 1619) but she did not have to pay dues to Denmark to use the Danish
Sound as other states had to.
The Russian War
This war was fought between 1611 and 1617. Russia, during these years, was going through what was known as "The Times of Troubles". Russia was seen by other Baltic states as being very weak (which she was). Sweden attempted to take advantage of this by blocking Russia’s attempted westward expansion towards the Baltic. At the same time she tried to stop any possible attempt by Poland to take over the Russian throne - such was the dislocation Russia was experiencing. A Polish occupation of Russia with the implied threat this meant to Sweden could not be tolerated by Gustavus. This was Gustavus’ first foreign policy test and he came out of it well.
Sweden’s military incursions into Russia were successful and gained for her a good settlement. Sweden gained Ingria and Karelia which linked Finland to Estonia (both Swedish possessions) and further strengthened her hand in the Baltic. But possession of both of these, also blocked Russia from advancing further west than Lake Ladoga and, as such, greatly reduced any threat to Sweden.
In this sense, Gustavus came out of this war well. It was his first real test. If Gustavus had failed in this war, it is possible that the nobles would have been less supportive of him. As happened, this was not to be the case.
The Polish Wars
These were a series of wars against Poland - a country that Gustavus was wary of for historic reasons.
The wars were fought from 1617 to 1618; 1621 to 1622 and from 1625 to 1629.
The king of Poland was Sigismund. He had been king of Sweden before he was deposed by Charles IX in 1599. Charles was the father of Gustavus.
The war had two purposes. First, Sigismund continual claim that he was the rightful heir of Sweden. Second, Gustavus wanted to capture ports on the Livonian and Prussian coasts in order to collect lucrative custom dues.
In 1621, Sweden captured Riga.
In 1625, Sweden captured Livonia.
In 1626, Sweden captured Memel, Pillau and Elbing - all cities of Polish Prussia.
In 1629, the Treaty of Altmark was signed, sponsored by Cardinal Richelieu of France. Richelieu wanted to free Sweden from any diversions to concentrate on the Thirty Years War. Under this treaty, Sweden kept all those ports whose income was more than the whole of Sweden’s revenue.
In foreign policy, Gustavus was successful. By his death in 1632, Sweden was the strongest of the Baltic states and was dominant in the area. Between 1611 and 1632, Gustavus had done a vast amount to advance the standing of Sweden in the region and in Europe in general. Whether it could be sustained throughout the Seventeenth Century was another issue.