 Complete the following phrase:  Mattenklotts division was given the objective of breaking through the defenses southwest of Nevrokop, proceeding further to the southwest until Serres and then turn to the north and attack Fort Roupel from behind, in order to capture the vital national road crossing the narrow valley known as the Rupel Pass. The invasion began on 6 April, and until the night, Mattenklotts troops had failed to punch through the Metaxas Line, suffering heavy casualties. However, during the following day, a breakthrough was achieved, but the advance towards Serres was retarded by the mountainous terrain. Most of the Greek forts continued to resist until 9 April, but as the main forces were isolated by the German advance to the west, they finally capitulated on the same day. Suming up his experiences from the battle, Mattenklott praised the Greek Army for its firm resistance and bravery. Following these developments, the XVIII Mountain Corps advanced until Thessaly. Other units captured the rest of the country, which came under total occupation with the 