Complex sentence: The Tallinn Offensive (Russian: Таллинская наступательная операция) was the Soviet offensive campaign fought over mainland Estonia in the Eastern Front (World War II) in 17--26 September 1944, corresponding to the German abandonment of mainland Estonia codenamed Operation "Aster".
Split into simpler sentences: The Tallinn Offensive (Russian: Таллинская наступательная операция) was the campaign fought over mainland Estonia in the Eastern Front (World War II) in 17--26 September 1944. The offensive corresponded to the German abandonment of mainland Estonia in the Operation "Aster".