The allied army saw success in the beginning,[4] but when Denmark declared war on Sweden, Charles abandoned the Transylvanians, which led to their defeat.

Although there are some disputes over the size of his force, it was not large and likely consisted of around 4,000 Swedes and 3,000 Brandenburgers divided into four regiments under Georg von Waldeck totaling to around 7,000 men.

[7] During this campaign, however, Charles had gained two new allies, being Prince George Rákóczi of Transylvania and Bohdan Khmelnytsky of the Cossack Hetmanate.

The latter having in February sent Colonel Anton Zhdanovich of Kiev with three Cossack regiments to link up with the Transylvanian Prince and the Swedish king.

[7] Rákóczi marched north into the Commonwealth with a large army split into three divisions under respectively John Kemény, Péter Huszár, and Ferenc Ispán.

A certain Gordon, who is a contemporary writer, claims that his army consisted of 20,000 Transylvanians, 5,000 Moldavians, 6,000 Cossacks, and 30 cannons, along with a supply train of around 1,000 wagons.

Not wishing to engage in a battle with the Transylvanians, Lubomirski lifted his siege and retreated towards the north-east, after this, Rákóczi relieved the Swedish garrison.

On the opposite side of the river, the Polish officers quickly ordered their men to cease their fire, greeted the Swedish king with all honours expected by a royal visitor, and promptly retreated.

[16] Despite being offered free departure from the city, a large part of the garrison, being around 600 men, instead decided to enlist in the Swedish army.

[20] The success of the allied siege was mostly because of Dahlberghs efforts, who had entered Brest in the disguise of a junior member of a negotiation team and managed to assess the fortifications.

Rákóczi could not be fully trusted either, as he refused to sign the treaties in which he and Charles had agreed to, and he carried our negotiations which he believed to be secret with Polish commanders, but this was exposed to the Swedes through their extensive intelligence network.

Rákóczi could do little on his own, and despite him accepting Charles' advice to abandon the campaign and return home, the Transylvanian army did not manage to survive July.