On November 30, after having broken off all diplomatic relations with Finland, the USSR attacked Finland in an undeclared war. The Soviet air force bombed Helsinki and other cities; the Soviet fleet bombarded Finnish ports; and the Red Army pushed across the frontier. The USSR employed a force of about 20 full divisions along many points of the 1,537-km (955-mi) frontier. Finland's standing army, which at the war's onset consisted of three infantry divisions, a cavalry brigade, and a tank company, was bolstered by trained and partly trained reserves. With these forces the Finnish general Baron Carl Gustaf Emil Mannerheim faced the Soviet attacking forces at five points. The first offensive of the Red Army was launched against a line of field fortifications and concrete pillboxes, called the Mannerheim line, on the neck of the Karelian Isthmus; the neck is 113 km (70 mi) wide. The Finns held the line. The four Soviet forces north of the Mannerheim line advanced during early December, but by January the Finns, aided by the intense cold, the forest-covered terrain, and the mobility of their ski troops and armament, cut the supply and communication lines of the Red Army and forced the retreat of its advance columns. In some cases the Russians were driven back beyond their own frontiers.