The military of the Swedish empire is commonly (and wrongfully) recognized only as the Caroleans, which were in fact not in service until the late 17th century under Charles XI and his successor.

Charles X Gustav was only king for 5 years, but conquered large amounts of territory that still belong to Sweden today (including Blekinge, Bohuslän, Skåne, and Halland).

The young king saw the need of a strong military force if Sweden was to survive as a nation, and thus he began reforming the army with inspiration from the strategies used by Maurice of Nassau.

Not only did the church-based conscription enable a quicker and easier way to find able men of suitable age, but religious influence would strengthen the morale and unity of the Swedish troops compared to foreign mercenaries.

It is believed that the army of Gustav II Adolf was the first military to utilize effective combined arms tactics in renaissance history, and that the Swedish Empire was indeed the most successful fighting force of the Thirty Years' War.

[7] The Swedish army at the beginning of the Thirty Years' War was equipped with state-of-the-art weaponry of domestic designs, including the leather cannon – a lightweight artillery piece that could fire at a fast rate and maneuver during the battle with only a handful of infantrymen (as opposed to the hostile artillery, which consisted almost entirely of enormous cannons that were very difficult to move even with horses).

But the Vasa did not pass its maiden voyage and sunk in 1628, leaving Sweden without its most powerful vessel and causing financial problems within the naval ministry.

The concept of Dominium Maris Baltici was not a primary goal for Sweden during the Thirty Years' War, as it was overshadowed by the struggle to protect Protestantism.

To do this he realized that he had to defeat the Holy Roman Empire on the field of battle; this was the dominating Catholic state in Europe which had begun enforcing its faith upon small Protestant nations in Germany, most notably Sweden's ally of Pomerania.

This action provoked Sweden to initiate an invasion, and after landing in allied territory the superior Swedish army easily defeated an Imperial force at Frankfurt one year after establishing a beachhead in northern Germany.

Sweden's limited manpower meant that from early on, the army had to rely on a majority of foreign soldiers, mainly from Germany, but also from Scotland or England (at least until the beginning of the English civil war).

In the beginning of the Great Northern War in 1700, Karl XII proved himself a gifted military leader and won devastating victories over his enemies with relentless offensive tactics in battles such as Narva, Düna, Kliszów and Jakobstadt.

Sweden saw no military failures in west or south, even though Russia had conducted some successful raids against Swedish outposts and villages on the eastern border.

Once the army had reached as far as Poltava, Ukraine, Charles himself was wounded and his men were in a dreadful state, having not slept or eaten for days and lacking sufficient ammunition.

Under the leadership of Field Marshal Carl Gustav Rehnskiöld the Caroleans desperately tried to overcome a heavily fortified line of defenses, but failed.

The Russian army severely outnumbered the Swedish invaders, was in good condition, had a much larger artillery force and were heavily dug in behind lines of redoubts along with wooden stakes and trenches.

The outcome of the events surrounding the disaster at Poltava effectively meant that Sweden had no land army left on continental Europe to contest renewed Russian attempts at reconquering their lost territories.