Complex sentence: After a short period of being in the front reserves on August 10, the 70th Army attacked a region to the north of Warsaw and by the end of August it reached Narew river near the town of Serock.
Split into simpler sentences: After a short period of being in the front reserves on August 10, the 70th Army attacked a region to the north of Warsaw. By the end of August, it reached Narew river near the town of Serock.