It took place between 850 and 853. Although only little territorial adjustments immediately resulted from it, its long-term consequences for the development of the Baltic region and Scandinavia were far-reaching.
Firearms reached Scandinavia and the Baltic region in the 800s (Sørstad, Älvsborg), 810s (Agder, Viken, Denmark, Geatland, Vineta) and 820s (Svearike-Sjonarike, Prussia, Courland, Karmøy, Trøndelag). Throughout the first half of the 9th century, the kings of the Svear in Uppsala had to witness a consolidation of Danish-Geatic-Sørstad hegemony Understandings between the Sørstad allies and the Frisian Hanse, forged with Celtic help, meant that almost no port towns were outside the reach of this duopoly. In the Far East, the strong Rome-backed Potamian Koina of the Borysthenes and the Rha had revoked privileges for Swedish traders after Svearike-Sjonarike`s defeat in the Vyatitch War, and strengthened commercial ties with the Hanse and the Sørstaders instead. Uppsala felt itself with its back against the wall.
Under these circumstances, king Gustav I forged an alliance with Hening, King of Saxony. He set Denmark-Geatland an ultimatum: Should they not have lowered their customs claims for the Kattegat passage by 30%, opened the port of Älvsborg for Swedish ships and guaranteed a safe, pirate-free passage through the Kattegat to Celtic Iutia with a member of the royal family as hostage in Uppsala by the spring aequinox in 850, then Sweden would enforce its free Kattegat passage.
The Danes and Geats had no intention of complying with the ultimatum. Älvsborgers activated their connections with Sørstad and affiliated towns like Vineta, Truso and Grobina. Pushed by the commercial associations and their representatives, all these town councils attempted to draw their military and political allies onto the Danish-Geatic side in the conflict. In the case of Sørstad, Vineta and Grobina, this worked well.
In the morning of April 22nd, 850, Swedish troops marched into Geatland, while simultaneously, Saxon troops landed on Western Danish islands. On April 28th, the Kingdom of Agder declared war against Saxony and Svearike-Sjonaike. Vineta and the Veleti Confederation followed on May 3rd, and Courland followed on May 7th. Only Truso`s councillors had not been able to persuade the Prussian chiefs to join the war against Denmark-Geatland.
All sides had tried to win the support of the Celtic Empire, or at least the Frisian Hanse, whose ships were the only ones in the Baltic endowed with Greek fire, but Celts and Frisians remained neutral, the latter selling weapons to both sides which the former had manufactured.
In the beginning, Denmark-Geatland was evidently over-strained with fighting on two fronts: While they were able to stop the Saxon advance in Fyn in May, they could not push the Saxon army back off Ærø and Langeland. Swedish divisions defeated the Geats in the battle of Skara and marched towards the Kattegat coast in June. But then, 16,000 soldiers from Agder arrived just in time to confront the Swedes in late June, who had lain siege on Älvsborg, which could not have withstood the cannons and the food shortage very much longer. The fights for Älvsborg continued intensely for 13 days, until the Swedish troops withdrew and dug themselves in in the hinterland on July 10th.
In the meantime, Vineta`s mercenary navy attacks the naval defenses of Saxon port towns in OTL Mecklenburg, while its Veletian allies lay siege to the towns and sack them. The Saxon King Hening is forced to withdraw his troops from Denmark in order to protect his own Baltic Sea settlements - but a large combined navy of Danes, Geats, Norse from Agder and Vinetans defeats the retreating Saxons and sinks their ships. 12,000 Saxon soldiers are trapped in Langeland and become captives of King Harald of Denmark-Geatland. The Saxon King Hening sues for a separate peace with Harald, offering Fehmarn in exchange for a safe return of his soldiers. Harald agrees, and the Saxons are marched and shipped back to Albingia.
On July 2nd, Curonian marine forces sacked and burned down Visby, the main port town of the Sjonar. The Swedish fleet pursues the Curonians, which is why neither side participated in the sea battle of Langeland.
Throughout July and August, the Saxons launch a counter-offensive to reconquer the port towns and fishermen`s villages on the Baltic Sea held by Vineta. Vineta`s navy withdraws tactically, but its Veletian allies are caught on the back foot. They lose ground against the Saxons every day. Towards the end of August, the Saxons are approaching Rethra, the central sanctuary of the confederacy. Just in time, though, Danish-Geatic forces break through Saxon naval coast defenses with Vinetan help and join with the Veletians to defend Rethra and stop the Saxon advance. The war between Saxony and Denmark-Geatland has begun again.
Hening must withdraw from the Slavic lands after losing the battle of Spandow (OTL Berlin) in September 850, but Saxons and Veletians merely gather forces for a new round next year.
In autumn, the Swedish fleet annihiliates its Curonian opponents in a sea battle near Tallinn. With its naval defense broken, the Swedes leap for Grobina - but the town`s fortifications withstand a whole month`s siege.
851 begins with Viken joining the Danish-Geatic side. This tips the balance in South-Western Sweden and forces Svearike-Sjonarike to withdraw from Geatic territories. It also prevents the Swedish King Gustav from launching a large-scale offensive in Estonia and Courland. Instead, he manages to convince the Prussian tribes of the Nadruvians and Bartians to side with him and march into Courland from the South.
As an immediate reaction, the Danish-Geatic-Vinetan-Viken-Agder-Grobina alliance declares war against these Prussian chiefs. In reaction, merchants from the Sørstad Alliance and those affiliated with them are expelled from Truso, many of them killed. Fights between Prussians and the Curonians` Aukshtatian and Zemaitian allies become entrenched and subside.
On the Western front, Saxony attempts a breakthrough in the South, but the Sorbs hold out with Corvatian support. Danes are still occupying Saxony`s North-Eastern port towns and secure them as best they can. The local population does not seem to oppose Danish rule.
Corvatia and the Veleti Confederation march on Prussia to assist their Curonian allies - or maybe just to secure their new sphere of influence. Caught between the Curonians and the Slavs, the Prussians must give themselves in. The Prussian chiefdoms are divided, those in the West become tributary to Vineta`s alliance, those in the South to Corvatia and those in the North are annexed into Courland.
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Almost all sides being financially and militarily exhausted, the war takes a break during the winter of 851/2. Only parts of Geatland, a couple of Danish isles and a few pockets of Veletian resistance around newly built fortresses in formerly East-Albingian Saxony are currently not in the hands of those who had controlled them in 849. The war could have stopped here - but the Swedish leadership had not yet given up on their claim to free access across the Kattegat, and, most importantly, the Celts did not yet it consider a quick end to this war as crucial for their interests.
While there were no major hostilities in Scandinavia in 852, the war raged on on the continent. In March 852, Saxony decided to strike at the heartland of Vineta and its Veletian allies and marched ion Rethra. This rallied all Veletian tribes behind Vineta´s campaign, though. A large Slavic army defeated the Saxons in a great battle at Spandow (OTL Berlin) in August 862, which went down into the history of the Northern Slavs as the great heroic defense of their homeland.
This would have been the perfect moment for Danish King Harald to intervene on the continent - but Harald waited so that the yeomen in his army could complete their harvests, and gathered his troops in late October. They landed on Fehmarn in November 852 and dug in. Saxon King Hening had had enough time to secure his defenses, When the Danes marched on Albingia, Saxony`s heartland, they encountered massive resistance. A battle near Pinneberg ended inconclusively.
In February 853, a series of separate negotiations restored peace between Denmark-Geatland and Saxony