Prussian campaign (1626–1629)

The Prussian campaign (1626–1629) was a Swedish invasion of Ducal Prussia during the Polish–Swedish War (1626–1629). Despite initial success, the campaign became a war of attrition and was inconclusive.

Swedish conquest of Livonia
After the Swedish victory at Wallhof, Gustavus Adolphus remained uncertain of the capabilities of his cavalry, the old defeats at Kircholm made the Swedish leadership unsure of how Swedish cavalry would perform against the Polish winged hussars. Nevertheless, the Battle of Wallhof was a Swedish victory, and it completed the Swedish conquest of Livonia.

Reason for the campaign
The Swedes had for long expected Danzig to be the port from where the Commonwealth would launch an invasion of Sweden. Gustavus recognized that if he were to move the war from Lithuania into Prussian, it would apply further pressure on Sigismund, since an offensive there would threaten his core territory.

An invasion into Prussia would also prevent Sigismund from assembling an invasion army there, Gustavus had already made plans for a war in Prussia in 1623–1624. Another reason was that the Swedes wanted to take advantage of the easier conditions for supply in Prussia, since the agriculture there had better yields than in Livonia.

Preparations
In late May 1626, Gustavus finally decided to shift the war into Prussia. The army had already been prepared, so was an invasion fleet consisting of 29 warships, 31 smaller vessels, and 81 transports. He also had 11,500 men, and 1,209 cavalry.

1626
In the last days of June, the Swedish fleet became visible to Pillau, a large bombardment from the fortress met the Swedish fleet, but the shots missed. The Swedish landings were executed in a calm manner. Pillau later surrendered, since its garrison of 340 men were not able to defend it. Gustavus explained to the people in Piillau that he took the city as a friend. Carl Gyllenhielm was later sent with a detachment of the fleet to Danzig to set up a blockade, and began to levy tolls. From Pillau, the Swedes began, against the electors wish, capturing many towns, including Braunsberg, Frauenberg, Elbing, Marienburg, Mewe, and Putzig. Negotiations regarding Danzigs surrender also began, but did not lead to anything.

Due to Danzig employing over 5,000 knights to its defense, the Swedes refused to initiate a standard siege, instead, Gustavus established a ring around the city and cut it off from supply. He also built two fortifications, one at Dirschau (modern day Tczew) and Danziger haupt (modern day Gdańska Głowa). In response to the invasion, Sigismund III organized an army in late July consisting of about 4,000 men, which was led by Stanislaw Koniecpolski. The army would later in September grow to a size of around 10,000 -11,000.

First Battle of Mewe
In September Sigismund would decide to recapture Mewe, and began marching towards it. It lacked a proper garrison, which only consisted of about 140 Swedes and 60 burghers. However, it was highly important to the Swedes, since it stood between the Commonwealth army and the Swedish camp at Dirschau. Gustavus was with the Swedish army at the time, which had suffered attrition during the siege of Danzig. On 11 September he began marching towards Mewe with around 2,000 men, and subsequent skirmishes took place at Warmhof, near Mewe. These skirmishes would come to be known as the First Battle of Mewe, but they were inconclusive. Afterwards, both sides received reinforcements, strengthening the Swedes to around 9,500 men.

Second Battle of Mewe
Meanwhile, the siege of Mewe continued, and the defenders were growing desperate. As a result, on October 1, Gustavus assaulted the Commonwealth camp, which became known as the Second Battle of Mewe. The Polish hussars charged at the Swedish infantry, but failed to make a significant impact. As a result of the battle, the Commonwealth siege of Mewe was lifted. Around 500 Poles and 30 Swedes would die.

Winter months
After the battles at Mewe, Gustavus would return to Stockholm, in spite of this, the fighting in Prussia continued. The Commonwealth forces attempted to push the Swedes from their fortified camp at Danziger Haupt; but Johan Baner, who was in command, repulsed all of their attempts to expel the Swedes. However, the experienced Polish commander, Stanislaw Koniecpolski, who had returned from the Ukraine with his troops, recaptured many of the fortresses, including Wormditt.

Battle of Hammerstein
In early April, the Swedish garrison was forced to surrender after a long blockade and siege. Later, Koniecpolski intercepted a Swedish relief force of about 2,500 men under the command of Maximillian Teuffel and Johan Streiff, the relief force had been marching towards Putzig to relieve it, only to find out that the Commonwealth forces had captured it earlier and were blocking the road. An attempt to bypass the Poles by the Swedes failed. Koniecpolski intercepted the remaining troops on April 12. Even though the Polish cavalry failed to break up the fortifications set up by the Swedes, after several skirmishes, the Swedes lost hope and mutinied. On April 15, they handed over both Teuffel and Streiff, after negotiations, most of the survivors entered Commonwealth service. Both Teuffel and Streiff were later exchanged and returned to Swedish service.

Assault on Käsemark
Gustavus returned to Pillau on 8 May 1627 with new reinforcements for the army and the Swedish army grew to 21,000, with the Commonwealth increasing their forces to 17,000. Despite this, the Poles were able to retain the field advantage, since the Swedes were forced to allocate a large portion of their forces to defending their camps. The Swedish situation had become complicated, while the Swedes remained entrenched in Danziger Haupt and Dirschau, two Commonwealth armies were deployed against them. The Swedes had imposed a blockade, but the Commonwealth forces had done the same to them. George William could not choose whose side to be on, but in the end, he chose not to support the Swedes.

Having resolved the situation with George William, Gustavus attempted to relieve the besieged camp at Danziger Haupt by carrying out a night-time amphibious assault on the Danzig controlled bastion of Käsemark opposite Danziger Haupt. Unfortunately for the Swedes, the sentries heard the Swedes approaching and opened fire. With the Swedes losing the element of surprise, the operation failed.

In a letter to John Casimir, his brother in law, who had become the regent of Sweden in Gustavus' place, he wrote:

"We went to the island of Hoefft, the enemy was just over against us on the Dantzic side, and began to play upon us with his cannon. Johan Baner and Count Thurn were to lead the first attack and I was to second them with the pikemen. We were all divided into our respective boats, and all would have gone well if my fellows had obeyed orders; but only one boat (Axel Duvall's) reached the opposite bank. The others mostly got stuck on a sand-bank, and one division of boats rowed in a wrong direction. So I jumped into a little boat to set the matter right. And because it is apt to get rather hot on such occasions, I was actually hit in the belly by a shot. But I have God to thank that it has not endangered my life or my health, and I hope in a few days to be able to direct the work again."

Battle of Morungen
The Swedish camp became worried when they were notified of a force of 1,800 Prussians that were sent by George William against them. Sigismund had earlier put pressure on him to intervene, either by recapturing Pillau or sending his troops to help Sigismund in battle. The two armies met at Morungen, the battle began and the Prussians quickly entered Swedish service, the Prussian troops would later form the Yellow Regiment.

Battle of Dirschau
After several more inconclusive attempts against Danzig, Gustavus began moving against Koniecpolski. The Swedes gathered at their old camp at Dirschau. Gustavus wanted to see how the Swedish cavalry held up against Commonwealth forces in a battle. On the 17 August the battle began, with the Swedish cavalry attacking the Polish hussars, since they had earlier began a retreat into the camp. The Commonwealth fought hard, but the Swedes were able to push them back. However, since night was approaching, Gustavus had to call off the battle for the next day.

The next day, Gustavus resumed the battle, since he knew that reinforcement for the Commonwealth forces were on the way. However, in the beginning, when he rode out to see how the battle proceeded, he was shot and wounded in the right shoulder. When night began approaching, the Swedes halted their advance. Nonetheless, this was the first battle where the Swedish cavalry decisively defeated the Polish hussars, and the Swedish army was no longer inferior to the Commonwealth one.

After the battle, Koniecpolski remained indecisive despite reinforcements from Wallensteins army. The years of campaigning also made the Swedish army more experienced, if these improvements had not taken place, it is unlikely that the Swedes would have made an impact in the Thirty Years War.

Battle of Oliwa
Danzig continued its resistance, despite the Swedish blockade, and the Commonwealth forces in Danzig contested it. On 28 November a battle took place off Oliwa near Danzig between 10 Commonwealth and 6 Swedish warships. The Swedes lost 2 ships, and the two admirals, Nils Stiernsköld, and the Commonwealth admiral, Arend Dickmann, fell in the battle. Due to the cold weather, Danzig was not able to exploit the victory.

End of the year
At the end of 1627, Sigismund was in a precarious situation. The one thing preventing him from making peace are the Catholic victories in the then ongoing Thirty Years' War. The war is not popular in the Commonwealth either, most of the people see the war as personal for Sigismund and that his claim to the Swedish throne is not a problem for Poland.

Battle of Weichselmünde
Gustavus Adolphus and fresh reinforcements arrived on May 15, 1628. The Swedish army in Poland is at this point consisted of upwards of 31,000 men, which was the largest army that Gustavus had ever commanded at that point. The Commonwealth forces had also been worn down to such a point that they were avoiding open battle, and focused on raiding the Swedes. and in late July, Gustavus carried out a surprise attack on Weichselmünde near Danzig. He brought 25 artillery pieces, all being leather cannons, across ground that Commonwealth forces thought was impassable. For several hours he carried out an artillery duel against the Commonwealth fleet. The Commonwealth navy lost 3 warships, but the remaining managed to take cover in Danzig's port and ensuing rain saved the fleet from further damage. The heavy rain would continue to August, imobilizing both armies.

End of the year
On the 8 August, after the rain had stopped, Gustavus once again set out to meet Koniecpolski in open battle, but Koniecpolski refuses and remains entrenched. Koniecpolski also enacted a scorched earth policy, which affected the Swedes heavily. The only battle would be the Battle of Osterode, where Koniecpolski successfully ambushed a newly recruited cavalry force under Heinrich von Baudissin. Gustavus also managed to capture a number of Commonwealth strongholds, with Swedish reconnaissance expeditions also passing by Warsaw. Having failed to defeat Koniecpolski, Gustavus handed over command to Herman Wrangel and returned to Sweden for the winter.

Battle of Górzno
In the beginning of the year, Herman Wrangel decided to reinforce the stores of Strassburg, which was besieged by the Commonwealth. On 30 January he set out with an army of 6,000 -8,000 towards Koniecpolski's army, which was made up of around 6,000 -7,000. Koniecpolski himself was not present, and the army was instead led by Stanisław Rewera Potocki.

The two armies faced each other on the 2 February, and the battle would begin on the left flank with cavalry under Maximillian Teuffel engaging with Polish cavalry. The German cavalry quickly begin retreating but the Polish cavalry remained and fought bravely. However, they would also quickly be overwhelmed and forced into a retreat. At the same time, the Commonwealth center would advance, but are quickly threatened by four Swedish cavalry squadrons. With this, the Commonwealth forces began retreating, with the Swedes following close behind.

Aftermath of Górzno
With the defeat at Gorzno, fear spread through the commonwealth, although the Swedes were not able to exploit the victory. The Sejm had however changed their attitude after the defeat, and allowed an Imperial army under the command of Hans Georg von Arnim-Boitzenburg to reinforce the Commonwealth. The effective strength of the army was around 16,000. Despite mistrust between them, Koniecpolski and Arnim worked together on a joint plan. On June 25, they united their armies at Graudenz, and they decided to cut Gustavus' communication lines to the north. At the same time, Gustavus would return to the front. He received intelligence on the joint Commonwealth-Imperial operations and correctly deduced that they were attempting to cut him off from the north.

Battle of Trzciana
After Gustavus set out with his army against the Imperial-Commonwealth force, he misjudged which route they were taking, he moved north towards Stuhm with his army, which then consisted of 1,300 foot, and 5,400 cavalry. The two armies met in Honigfelde (modern day Trzciano, which the Swedes misjudged to be Stuhm.) Gustavus quickly ordered a delaying action to allow time for his troops to retreat, their situation was critical, but nonetheless, Gustavus managed to save most of his units. One notable loss is Gustavus' hat, which the Imperials took as a war trophy. Despite the successful retreat of most of the Swedish units, the battle is considered a Swedish defeat. Gustavus later described the battle in the following words:

"'Never have I experienced a hotter bath!'"

Aftermath of Trzciana
After the battle, the Prussian campaign ended in a strategic stalemate, with either side being able to inflict a decisive victory on the other, and distrust started building in the Imperial-Commonwealth army, with Sigismund not trusting the Imperials, and the Imperial army being short on everything. On August 19, the Commonwealth forces would depart from their fortified camp, and the Swedes managed to successfully assault the Commonwealth rearguard, capturing 300 wagons, along with the entire army's gunpowder stockpile, this would turn out to be the last battle in the war.

Aftermath
In September 1629, Sweden and the Commonwealth signed the six-year long Truce of Altmark, this had been pushed by England, France, and the Dutch Republic. It could be no more than a truce, since Sigismund refused to relinquish his claim on the Swedish throne.

Results of the truce
As a result of the truce, the Swedes gained de facto control of Livonia north of the Daugava, they also retained control of the important Prussian coastal towns of Braunsberg, Elbing, and Frauenberg, in addition, the Swedes received the east Prussian towns of Pillau, Fischhausen, Lochstädt, and Memel. More importantly, the Swedes also acquired the right to the main share of the tolls levied on the Danzig trade.