Swedish intervention in the Thirty Years' War

The Swedish invasion of the Holy Roman Empire, or the Swedish Intervention in the Thirty Years' War, that took place between 1630 and 1635, was a major turning point of the war often considered to be an independent conflict. After several attempts by the Holy Roman Empire to prevent the spread of protestantism in Europe, King Gustav II Adolf of Sweden ordered a full-scale invasion of the Catholic states. Although he was killed in action, his armies successfully defeated their enemies and gave birth to the Swedish Empire after proving their ability in combat. The new European great power would last for a hundred years before being overwhelmed by numerous enemies in the Great Northern War.

Background
The "Roman Catholic States" led by the Catholic Habsburg Imperial family sought to tighten its grip on the Holy Roman Empire, and gain formal in addition to retaining its de jure control over the Empire, which it had by and large secured by this point, albeit still on risky foundations. The political structure of the Holy Roman Empire ensured they would never seize this as a hereditary right unless the political structure of Germany was altered. They sought to do this by suppressing the practice of the Protestant branch of Christianity in north Germany, and reseating the Protestant potentates in the Empire with Catholic ones. This was because election to the Holy Roman Empire was divided between a number of princelings who were of both Protestant and Catholic faith. They simultaneously seized Protestant places of worship and started to roll back many of the religious compromises that had previously been arranged in order to preserve religious stability within the empire. This would afford them the opportunity to alter the political structure of the Holy Roman Empire via its legislative body. The Protestant powers were fighting to preserve their religious freedom, and simultaneously preserve their political autonomy from the Habsburgs.

The Holy Roman Empire was a hodgepodge of Protestant and Catholic states of various sizes that exercised sway over Germany, and over European politics, according to the resources and armies that they had at their command. Broadly speaking, the Holy Roman Empire – prior to the outbreak of the Thirty Years' War – was divided in half between the Protestant North and the Catholic South.

The Swedish situation
Gustav II Adolf of House Vasa had inherited the throne at age sixteen in the year of 1611. He immediately showed an interest in reforming the military, seeing little value in the common feudal warfare of peasant draftees. With his reorganizations came a professional standing army of 22,834 men (19,584 infantry and 3,250 cavalry), equipped with the most advanced weapons available and drilled to become a superior fighting force. However, an issue remained that the territories under Swedish control were large but held small populations, making the king favor a strategy of counterattack rather than defense, should any other nation attack Sweden. Gustav II Adolf was confident in the Swedish military superiority as of 1630, and personally commanded his men into battle and won great victories against much larger enemy forces. The Royal Swedish Navy consisted of 22 ships and it outclassed the embryonic Imperial fleet that Wallenstein tried to build up in Stralsund.

Battles
At the time of the Treaty of Stettin, Sweden had one of the best equipped and trained military powers in the world and were theoretically superior to the Holy Roman Empire's soldiers by far. However, unlike the Imperial army which had been fighting against protestant German states for 12 years, the Swedes had no experience of actual combat and had to resort to risky operations that could very well have caused the destruction of the Swedish army. In the first battle at Frankfurt an der Oder, the Swedish army successfully defeated an entrenched Imperial force with pure offensive power and Gustav II Adolf would repeat the same pattern for the rest of his life: a quick, powerful offensive with superior discipline and mobility to overwhelm the enemy. The Swedish military superiority over the Holy Roman Empire, considered a near-superpower at the time, was made obvious in battles such as Breitenfeld and Oldendorf. Despite the death of Gustav II Adolf at the otherwise successful battle of Lützen, the Swedish army continued to press forward into catholic territories with amazing success until the peace of Westphalia was signed in 1648. Sweden's rise into a European great power had been confirmed, and the Holy Roman Empire's power begun to decline with their defeat in the Thirty Years' War, mainly due to the Swedish efforts. Of all parties in the conflict, the Swedes were the most successful and claimed responsibility for the loss of more than 100,000 Imperial troops, which was half of the national army.