The Commonwealth, focused on a war with the Ottoman Empire (such as the battles of Cecora and Chocim), was unable to send significant forces to stop Gustav Adolf, and signed a truce favorable to Sweden.

The Commonwealth ceded Livonia north of the Dvina (Düna) river, and retained only nominal control over Riga.

With help from Axel Oxenstierna, the Swedish king introduced a widespread program of military and social reforms, which resulted in the creation of a well trained army, based on native recruits.

The invaders, marching through marshes and forests to avoid Lithuanian cavalry, entered the Duchy of Courland and Semigallia, capturing its capital, Mitau without resistance.

On 7 January Radziwiłł recaptured Mitau, but because of a lack of artillery, he failed to seize its castle, whose garrison did not capitulate until 6 July.

The truce, signed by the Lithuanians, enraged King Sigismund III Vasa, who planned to transfer all forces northwards, hoping that with the help of the Spanish Navy, he would be able to invade Sweden itself.

Radziwiłł emphasized the fact that after the reforms, the Swedish army was difficult to defeat, and Poland–Lithuania also needed a major overhaul of its armed forces, especially the infantry and artillery.

Regardless of the opinion of his generals, Sigismund III ordered construction of a fleet, which, with help from the Spanish Empire, was to transport the invading army to Sweden.

However, from the very beginning, this was questioned by the powerful and semi-independent city of Gdańsk, whose government, under Swedish pressure, opposed the very idea of a Commonwealth fleet, and did not allow it to be anchored in its port.

They were manned mostly by local Kashubian fishermen, while marine infantry consisted of English mercenaries, employed by the Polish King since 1621.

[5] Before the end of the truce, Gustav Adolf initiated talks with the Tsardom of Russia and the Ottoman Empire, hoping for a joint attack on Poland–Lithuania.

This success cut links between Polish–Lithuanian garrisons remaining in Livonia, and those stationed in Lithuania, as Sweden now controlled the line of the Daugava.