Complex sentence: Contacts with both groups were severed by the first Axis winter offensive, but German decrypts showed that the Chetniks were collaborating with the Italians on the base of an old friendship of Serbs and Italians in Dalmatia going back to the times of the Autrian rule.
Split into simpler sentences: Contacts with both groups were severed by the first Axis winter offensive, but German decrypts showed that the Chetniks were collaborating with the Italians. This collaboration was based of an old friendship of Serbs and Italians in Dalmatia going back to the times of the Autrian rule.