Complex sentence: But those areas were also very easy to encircle if counterattacks were not materialized, which were shown only a year later, in June 1941, when hostilities resumed in the Continuation War.
Split into simpler sentences: But those areas were also very easy to encircle if counterattacks were not materialized, which was soon to be shown. Only a year later, in June 1941, hostilities resumed in the Continuation War.