As Charles X appeared to be fully occupied in Poland, Frederick III judged the time appropriate for the recapture of the other Danish-Norwegian provinces.
Reacting swiftly, by forced marches Charles X brought his hardened armies from Prussia to Holstein.
The terms were brutal: Then Charles X ignored the recently negotiated Treaty of Roskilde when he invested in Copenhagen in August 1658.
His goal was to recapture Trøndelag and to defend the Norwegian border at Halden, which Charles X had demanded be turned over to Sweden as it provided both an excellent port for timber export from the newly acquired Bohuslän and a point from which further invasions could be launched.
In early January 1660, the Swedish forces again attacked Halden; it was to serve as the base for their advance on Akershus in Oslo.
Under heavy bombardment the inhabitants begged the commandant to surrender, but putting his faith in his garrison, Huitfeldt held on.