Complex sentence: Once again, the Red Army found itself on the defensive in Belorussia, however the Poles were able to successfully break the Russian lines at the Battle of the Niemen River in September 1920.
Split into simpler sentences: Once again, the Red Army found itself on the defensive in Belorussia. The Poles were able to successfully break the Russian lines at the Battle of the Niemen River in September 1920.