The Swedish invasion that followed came close to causing the complete disappearance of Poland from the map of Europe. All that prevented a total Polish capitulation to its Swedish and Russian neighbours was, in my view, the underestimation of the Polish spirit, and the gross failure to assess the damage to its reputation that would follow from making war feed war, a strategy which required plundering and pillaging to keep Swedish finances afloat.
The conquering of Poland proved to be an impossible objective though; despite the seemingly overwhelming losses suffered by the Poles, the Swedes were unable to occupy all of the country and make it their own. Their constant public relations faux pas enraged Polish opinion, while increasing foreign concern led to a gradually decreasing Swedish presence in Poland in 1656. By the following year in 1657 Charles Gustav had turned his attentions to the next enemy to join the anti-Swedish league; Denmark.
Yet Charles had also spent most of 1656 simultaneously trying to buy the favour of his Brandenburg neighbour. Since he his successes had alienated much of his potential allies, Brandenburg became gradually more indispensable to Sweden as 1656 progressed. By the end of the year 3 treaties had been signed with the Brandenburgers, but it proved inadequate for Brandenburg's Elector, Frederick William. Frederick desired not just nominal possession over Ducal (East) Prussia, but total sovereignty, and only Poland, as Ducal Prussia's overlord, was able to grand this. Thus in September 1657 did Brandenburg officially become Brandenburg-Prussia, as the price for buying the Great Elector's favour and causing him to declare against the Swedes was a promise of the right to inherit and rule over Ducal Prussia, with no strings attached. Sweden's Pomeranian holdings were now seriously threatened. Thus Brandenburg had been flipped, and Sweden was truly alone.
The flipping of Brandenburg occurred in the background of the invasion and occupation of the Jutland Peninsula by the Swedes, which left the Danish King Frederik III holding out in Copenhagen, hoping that the fortress of Frederiksodde would hold the Swedes on the Peninsula while the anti-Swedish alliance mustered against him. Yet, while such issues seemed promising good fortune struck for Sweden; the Danish Straits froze over following a harsh winter and thus provided a stellar opportunity for Charles Gustav to march across them, thus removing the major Danish defence and granting Sweden a serious opportunity to end the war on decisive terms; the most decisive the Danes had ever experienced.
The Treaty of Copenhagen brought home the totality of Danish defeat and the supremacy of Sweden over its former enemies. Forced upon its Danish enemy by a Swedish army on its home island, Denmark was stripped of any possibility of ever reclaiming its old status. Never again could European statesmen perceive Denmark as anything but a second rate power, and never again could the Baltic have any other overlord but Sweden. The Vasa state had triumphed over Poland on a level never before imagined possible, and here now it had done the same against its Danish foe. Sweden had trounced its greatest enemies, but the war still raged on.
For the anti-Swedish league, the defeat of Denmark was a blow to their hopes of distracting Sweden long enough in Scandinavia to be able to deliver a knockout blow in Northern Europe. Sweden had been mostly driven out of Poland, but it had left behind an alliance of Transylvanians and Cossacks that kept the alliance of the Poles and Habsburgs busy for much of 1657. Only in 1658 was anything resembling an answer to Swedish dominance forming, but the Danish loss was a tough one to take for the beleaguered Holy Roman Emperor and his Polish ally who, though joined by Brandenburg, now had to face a confident, empowered Sweden secure in its western flank and out for revenge; Frederick William of Brandenburg felt sure that his lands would be on the list of places Charles Gustav would visit.
The news for the anti-Swedish league grew worse when Russia re-entered the war against the Poles and signed a truce with the Swedes, freeing up both more Swedish forces in the east and further pressuring the Poles. The issue for Russia proved to be the creation of the Union of Kėdainiai, which would have created a Polish-Lithuanian-Ukrainian Commonwealth and would have violated the earlier Treaty of Pereyaslav that Russia had signed with the Cossacks which had pledged the latter to the Tsar's Empire. The renewal of the Russo-Polish war was but another complication in the series of struggles that the Second Northern War had now degenerated into.
For Charles Gustav, the threatening stance posed by Dutch and Danish friendship partly inspired him to ensure a tougher stance be taken against the helpless Danes in the treaty negotiations that took place over the early summer of 1658. However, once it became clear that the Danes were dragging their heels, the Swedes broke off negotiations and attacked the Danes again in early autumn. This time, since the Swedes had attacked first, the Dutch could honour their defensive alliance, and a Dutch fleet did battle with the Swedes, freeing the siege of Copenhagen in early 1659 and paving the way for a dogged Danish defense as Frederik III of Denmark inspired his people to resist. With enemies on all sides of him and Brandenburg leading the allied armies, Charles Gustav had quite the struggle ahead. It appears to have taken its toll, because upon falling ill while visiting troops in early 1660, he died shortly thereafter, plunging the Swedish Empire into another Regency and paving the way for peace out of necessity.
The following treaties brought peace at last to Northern Europe. However, for all intents and purposes too much had changed in the five years of war for any European statesmen to view the region in the same way again. The old ambitions of once great powers; Denmark and Poland, now clearly lay in ruins owing to their overwhelming defeats, while Sweden was the undisputed military masterclass of Europe and its empire was further extended. But at what cost? Sweden remained plagued by internal problems, as well as hounded by opportunistic enemies like the rising powers of Brandenburg-Prussia and Russia. Sweden had alienated itself from Europe because of its conquering ways, and its empire remained no easier to keep unified than before, requiring large garrisons in every corner of it to keep it all together. The impression Sweden made on the map was monumental, but Charles Gustav had much to do before his untimely death further kicked the can down the road. It would be the Swedish misfortune in later years that Swedish policymakers, in their desperation, turned to the only ally they felt they still had following their unleashing of deluges upon Europe: Louis XIV's France. It was to be a partnership that almost collapsed the entire Vasa state.
Undoubtedly impressive, the Swedes had cemented their military reputation and could at least bask momentarily in their glory. With a regency looming and an uncertain economic future though, it would take a king with more than the talents of Charles Gustav to protect all that had been taken in the years of warfare known as the Swedish Deluges.
Thanks for listening, and see you all once my thesis ends!