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.

[8][7] 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.

[9] Carl Gyllenhielm was later sent with a detachment of the fleet to Danzig to set up a blockade, and began to levy tolls.

[9][7][11] From Pillau, the Swedes began, against the electors wish, capturing many towns, including Braunsberg, Frauenberg, Elbing, Marienburg, Mewe, and Putzig.

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.

[7][9] 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.

[7][9] However, the experienced Polish commander, Stanislaw Koniecpolski, who had returned from Ukraine with his troops, recaptured many of the fortresses, including Wormditt.

At the beginning of November 1626, the kwarciane army under the command of Crown Field Hetman Stanisław Koniecpolski arrived in Royal Prussia.

The Hetman intended to use the winter season to carry out several military operations, one of which aimed to remove the Swedish garrison from the war port in Puck.

[13] The first attempt ended in partial success, as the Poles managed to take the town, but the attack on the heavily fortified port failed.

By the end of March 1627, Koniecpolski, taking advantage of the spring floodwaters of the Vistula, which made offensive operations difficult for the Swedish troops, set out for Puck again.

A siege lasting several days, supported from the sea by artillery fire from the Polish fleet led by Arend Dickmann, finally bore fruit, as part of the fortifications were destroyed, forcing the Puck garrison to capitulate.

[13] All cannons, ammunition, and loot plundered by the Swedes in the Commonwealth fell into the hands of the Crown army.

[9] The Swedish situation had become complicated, while the Swedes remained entrenched in Danziger Haupt and Dirschau, two Commonwealth armies were deployed against them.

[7][9] 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.

The one thing preventing him from making peace were the Catholic victories in the then ongoing Thirty Years' War.

He brought 25 artillery pieces, all being leather cannons, across ground that Commonwealth forces thought was impassable.

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.

[7][9] Gustavus also managed to capture a number of Commonwealth strongholds, with Swedish reconnaissance expeditions also passing by Warsaw.

[9] Having failed to defeat Koniecpolski, Gustavus handed over command to Herman Wrangel and returned to Sweden for the winter.

[7][9] In the beginning of the year, Herman Wrangel decided to reinforce the stores of Strassburg, which was besieged by the Commonwealth.

[9] 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.

On June 25, they united their armies at Graudenz, and they decided to cut Gustavus' communication lines to the north.

He received intelligence on the joint Commonwealth-Imperial operations and correctly deduced that they were attempting to cut him off from the north.

[9] 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.

[7] 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.