On 12 September 1944, King Peter II called on all Yugoslavs to come together under Tito's leadership and stated that those who did not were "traitors", by which time Tito was recognized by all Allied authorities (including the government-in-exile) as the Prime Minister of Yugoslavia, in addition to commander-in-chief of the Yugoslav forces. On 28 September 1944, the Telegraph Agency of the Soviet Union (TASS) reported that Tito signed an agreement with the Soviet Union allowing "temporary entry" of Soviet troops into Yugoslav territory which allowed the Red Army to assist in operations in the northeastern areas of Yugoslavia. With their strategic right flank secured by the Allied advance, the Partisans prepared and executed a massive general offensive which succeeded in breaking through German lines and forcing a retreat beyond Yugoslav borders. After the Partisan victory and the end of hostilities in Europe, all external forces were ordered off Yugoslav territory.
Who executed a massive offensive and succeeded in breaking through German lines?
the Partisans