The term Dominium maris baltici refers to the establishment of a Baltic Sea dominion. After, the Kalmar Union broke up in 1523 establishing a Baltic dominion was one of the main goals of the now independent Swedish nation. Sweden would came very close to achieving its goal, but the Great Northern War (1700-1721) put an end to its ambitions. However, during its height the Swedish Empire was one of the greatest powers in Europe. Traditionally, the empire began during Gustav II Adolf's reign. Gustav II Adolf of the House of Vasa ( also known Gustavus Adolphus) ascended to the throne in 1611 and throughout his reign made notable victories for Sweden, despite inheriting three wars from his father (against the Danish, Russians and the Polish-Lithuanian Commonwealth). He ended Russian access to the Baltic with the Treaty of Stolbovo (1617) and he gained the province of Livonia from the Polish after the Truce of Altmark (1629). The conflict with the Danish was less successful, after Sweden was forced to pay a large ransom for the Fortress of Alvsborg, when the Treaty of Knäred was signed in 1613.
Gustavus Adolphus changed the course of the Thirty Years War (1618-1648) when he intervened in 1630. His intervention may have been for economic gains and the defense of the Protestant faith,which was being suppressed by the Habsburgs, throughout the Holy Roman Empire. Gustavus Adolphus won a great victory at Breitenfeld, in 1631, the victory ensured Sweden's continuation in the Thirty Years War and it hampered Imperial and Catholic forces. However, the reign of one of Sweden's greatest kings, came to quick end at the Battle of Lützen (1632), when he died in battle. Despite, his death the battle was a Swedish victory and the Swedish Empire would continue to expand for the years to follow.
The Swedish throne was inherited by Gustavus Adolphus' only legitimate child Christina. However, she was underage, so Maria Eleonora (Adolphus' wife) and the King's ministers assumed control of the government, until she took control in 1644. She also gained several possessions throughout her reign by seeking peace with Denmark. The Peace of Westphalia (1648) ended the Thirty Year's War and it considerably favoured Sweden. The treaty gave Western Pomerania to Sweden and changed its name to Swedish Pomerania. The treaty also gave other territorial gains to Sweden and 5,000,000 Riksdaler. However, the Peace of Westphalia did not bring an end to the struggle for control over the Baltic. Sweden would remain a great power and an important political player in Europe for many decades to come.