Gustavus Adolphus (9 December [N.S 19 December] 1594 6 November [N.S 16 November] 1632), also known in English as Gustav II Adolf or Gustav II Adolph, was King of Sweden from 1611 to 1632, and is credited for the rise of Sweden as a great European power (Swedish: Stormaktstiden). During his reign, Sweden became one of the primary military forces in Europe during the Thirty Years' War, helping to determine the political and religious balance of power in Europe. He was formally and posthumously given the name Gustavus Adolphus the Great (Swedish: Gustav Adolf den store; Latin: Gustavus Adolphus Magnus) by the Riksdag of the Estates in 1634.He is often regarded as one of the greatest military commanders in modern history, with use of an early form of combined arms. His most notable military victory was the Battle of Breitenfeld in 1631. With his resources, logistics, and support, Gustavus Adolphus was positioned to become a major European leader, but he was killed a year later at the Battle of Ltzen. He was assisted in his efforts by Count Axel Oxenstierna, the Lord High Chancellor of Sweden, who also acted as regent after his death.

Coming to the throne at the age of 16, Gustavus Adolphus inherited three wars from his father Charles IX of Sweden; border conflicts with Russia and Denmark-Norway, and a dynastic struggle with his first cousin, King Sigismund III Vasa of Poland. Of these, the Danish war was the most serious. During his reign, Sweden rose from the status of a Baltic Sea basin regional power to one of the great powers of Europe and a model of early modern era government. Gustavus Adolphus is known as the "father of modern warfare", or the first modern general. He taught a number of other military commanders, such as Lennart Torstensson, who would go on to expand the boundaries and the power of Swedish Empire after Gustavus Adolphus's death. Spoils meant he became a successful bookraider in Europe, targeting Jesuit collections.His contributions to Sweden's rise in power included reformation of the administrative structure. For example, he began parish registration of the population, so that the central government could more efficiently tax and conscript the people.He is widely commemorated by Protestants in Europe as the main defender of their cause during the Thirty Years' War, with multiple churches, foundations and other undertakings named after him, including the Gustav-Adolf-Werk.

The Thirty Years' War was a conflict fought largely within the Holy Roman Empire from 1618 to 1648. Considered one of the most destructive wars in European history, estimates of total deaths caused by the conflict range from 4.5 to 8 million, while some areas of Germany experienced population declines of over 50%. Related conflicts include the Eighty Years' War, the War of the Mantuan Succession, the Franco-Spanish War, and the Portuguese Restoration War.

Until the 20th century, historians considered it a continuation of the German religious struggle initiated by the Reformation and ended by the 1555 Peace of Augsburg. This divided the Empire into Lutheran and Catholic states, but over the next 50 years the expansion of Protestantism beyond these boundaries gradually destabilised Imperial authority. While religion was a significant factor in starting the war, scholars generally agree that its scope and extent was driven by the contest for European dominance between Habsburgs in Austria and Spain, and the French House of Bourbon.The war began in 1618 when Ferdinand II was deposed as King of Bohemia and replaced by Frederick V of the Palatinate. Although the Bohemian Revolt was quickly suppressed, fighting expanded into the Palatinate, whose strategic importance drew in the Dutch Republic and Spain, then engaged in the Eighty Years War. Since ambitious external rulers like Christian IV of Denmark and Gustavus Adolphus of Sweden also held territories within the Empire, what began as an internal dynastic dispute was transformed into a far more destructive European conflict.

The first phase from 1618 until 1635 was primarily a civil war between German members of the Holy Roman Empire, with external powers playing supportive roles. After 1635, the Empire became one theatre in a wider struggle between France, supported by Sweden, and Spain in alliance with Emperor Ferdinand III. This concluded with the 1648 Peace of Westphalia, whose provisions included greater autonomy within the Empire for states like Bavaria and Saxony, as well as acceptance of Dutch independence by Spain. By weakening the Habsburgs relative to France, the conflict altered the European balance of power and set the stage for the wars of Louis XIV.