This timeline revolves around the king of Sweden, Gustav Adolf II (Gustavus Adolfus is what he's commonly called outside of Scandinavia) not dying in a cavalry charge at Lutzen (1632). It also has minor changes to OTL (Bernard of Saxe-Wiemar dies two years early, Tortenssen isn't delayed by Hansiatic League traders when he take the army across the Baltic, etc). It starts off in the Thirty Years War, pitting primarily Catholic southern Germany, against primarily Protestant northern Germany. Sweden was the main superpower among the protestants, while the Austrian Habsburgs (or Imperials) were the main Catholic power.
The Landing at Rostock (1625-1626)
Under orders from Gustav, Tortenssen takes the entire standing army into Rostock in 1625, in order to put Germany firmly into the Protestant sphere of influence. (This happened OTL in 1630, because of resistance by traders). This force was about 30,000 men, most of them ethnic Swedes and Finns. They were very different from the Catholic armies for a number of reasons. Unlike the Imperials, it was completely religiously homogeneous, being 60/40 Lutheran/Calvinist. It was also ethnically similar, all of them hailing from the Swedish Empire. Thirdly, it was one of the first standing armies, making it infinitely better than the predominately mercenary Catholic armies (I'm going to refer to the Catholics as imperials, or as Hapsburgs. Just know when I say Swedish Empire it's different from the Imperials.). Lastly, they had simply amazing artillery. Their gunnery and cannons were average, but Gustav had standardized the size and weight of the ammunition and guns. This simplified the supply train considerably, as the Imperials had to have ammunition crafted for individual cannons.
The standing force was intended only as a core of the army, as German mercenaries would be in great supply in Germany. After borrowing money from the burghers of Rostock, Tortenssen managed to hire 20,000 troops. With these, he marched straight up to the Duke of Pomerania, forcing him to sign the Union of Rostock. This gave Sweden complete control over Pomerania, and a useful supply base. (OTL, the Swede's supply train stretched all the way back to Stockholm). Tortenssen sent Horn with 4000 Finnish cavalry and several batteries of artillery to make similar deals with Wismar, Mecklenburg, Hamburg, and Bremen. Supported by the navy, these cities capitulated within a year. This gave Gustav control over the entire northern coast of Germany.
Brandenburg had long been a bastion of Calvinism in Germany. It had also long been a bastion to the 7000 Imperial mercenaries under the Count of Tilly. As soon as Tortenssen had landed, he sent troops into the surrounding countryside to keep the Imperials from foraging. As Horn returned from Bremen with 19,000 German Calvinist volunteers, he lay siege to the city. The mercenaries capitulated in two weeks due to the strong resistance the Protestant city folk were giving them.
By now, Sweden controlled the northern one-fifth of Germany. (Draw an imaginary line East-West going straight through Bremen on Google Maps (I'll have actual maps soon). However, this required money, and Sweden was in need of it. Gustav saw an amazing opportunity, and he took it. Under the Edict of Gotland, he would allow freedom of religion on the island. Huge numbers of Jews immigrated there, bringing huge amounts of wealth and trade. As the Jewish community were the main bankers and moneylenders for European monarchy, Gustav now had some very friendly, very rich bankers living on his doorstep. This enabled him to pursue a new strategy in his fight.
The Frankfurt Campaign
This would be the first use of asymmetric warfare in Europe. Ljus Spadbarn (light infantry) units deployed around the countryside. used in company-sized units, they would completely ignore cities and towns, moving through villages and working with farmers in order to secure their support of the operation. The Imperials were so taken aback bt this strategy that initially they tried to pursue the Swedes in tercio formation. Through ambushes and taking down bridges, they managed to confine the Imperial forces west of the Elbe into an urbanized triangle of Frankfurt, West Baden, and Darmstadt. Most of the Imperials got their daily bread and butter through "foraging" (glorified looting), so being in cities didn't really help that. Imperials found it impossible to gather food in the nearby farms, mainly because the light infantry were covering the countryside. Sniping as we know it today was impossible, due to rifles not being invented yet, but the Imperials would forage in small groups of friends, usually five to seven. With 10-15 well trained Swedes taking them by surprise, they wouldn't stand a chance. The Ljus Spadbarn were only about 10,000 to 15,000 strong, so support from the main army was needed. A circle 80 miles in diameter around the triangle was fortified, and the Swedes under Horn with 20,000 Swedish line troops, 12,000 Gothlander, Finn, and Sami cavalry, 15,000 German tercio forces, and 30,000 mercenaries laid siege. As the commander of Imperial forces in Frankfurt, Bernard of Saxe-Wiemar rode out to check on his forces efforts to fortify against the coming assault. He was struck by a mortar shell, mortally wounded, and died the next day due to bleeding. The mercenaries, leaderless, surrendered the following day. The Hapsburgs had lost all territory except the Spanish Netherlands (OTL Belgium).
A New Union (1627-1628)
The Swedes had rolled up an incredible amount of territory in a very small amount of time, and the patchwork of principalities and city-states that they controlled were beginning to rethink their allegiances. Gustav needed a solution fast, and his aide-de-camp, Axel Oxensternia, provided the answer. He proposed that, in order to continue the war on the Hapsburgs, they would need a permanent political settlement with Germany. This would keep the various dukes and princes satisfied, while enabling Sweden to draw on a huge manpower pool for further conquest. The deal was presented to the princes at Magdeburg in 1627. It was debated back and forth for three months, until finally Gustav simply proclaimed it and ignored the inevitable complaints. Under it, all the German territory would be ceded to Sweden, but the local rulers would retain nominal control over the territory. As in Sweden, all the boys between the ages of 18-24 would have to serve in the military, and a national tax would be levied. The internal affairs of Swedish Germany (the official title) would be governed by democratically elected officials that would come together at the House of Lords in Magdeburg to discuss issues. During this time, the Swedish troops were busy securing the conquered territories.
The Peace of Lorraine (1627)
Gustav and Emperor Maximilian declared a peace while meeting in Lorraine. All German territories outside of Austria would be added to S. Germany, however, the Spanish Road would be kept open for Austrian troops and supplies to go to the Spanish Netherlands.
Immediately after the peace, Gustav ordered the College of Jena (a well-respected establishment located in Germany. Much like Oxford or Cambridge of a later era) design a musket that could fire 15 shots in ten minutes, be able to be mass-produced in numbers large enough to completely rearm the Swedish military in less than five years, and be able to be scaled down for use by cavalry or by marines.
1629: The Ministry of Colonisation is established. As the 30 Years War (ATL called the German War) had displaced hundreds of thousands of civilians, from the Netherlands to Poland, a chance to start a new life, free from almost all restrictions, was too good to pass up. Each family would be given free passage to the New World, a small amount of money (OTL equivalent: $100,000), and 100 acres of mixed forest and grassland. Taxes would be half as much as they would be in the Old World; however, the colonists would have to be able to pay off their debt within ten years, or be evicted and their land given to new colonists.
1633: The College of Jena presents it's musket. It is approximately 11 pounds, and can fire (on average) 16 rounds every ten minutes. Powder and shot would be contained together in a linen cartridge. When rammed down, small blades at the muzzle would slice the cartridge open, pouring the powder down. Everything else was the same as any other musket, except for the fact that the snaphaunce could be cocked back by pulling on the trigger guard, and the fact that the powder horn had a small lever to measure out exactly the right amount of primer. It was adopted as the Universal Service Rifle (mark 1). The musket was first assigned to the fusiliers (troops tasked with protecting the Swede's amazing artillery).
1634: Two states are added to the Swedish Union, adding to the existing 11. These were New Sweden (Nova Scotia through Connecticut, with all territory east of the St. Lawrence river), and Hejiskii (New Jersey south to northern Florida, and west to the western Appalachians.). As opposed to OTL, the French were prevented from settling Canada, since the Swedes took all the coastline, and were using fishing boats to patrol the waters. The Dutch, being allies of the Swedes, held on to OTL New York (ATL New Amsterdam).
The King's English War (1634-1639)
Due to the Swede's rapid colonisation of The Americas, England began to think of taking its colonies. However, it would have to transport troops to the Americas, and, with only a small enclave near Jamestown, had no bases in the area. This being so, England decided to attack Sweden in Europe.