Complex sentence: He did not finish the "gimnaziya" but volunteered for the army in 1915, serving in the artillery; after the October Revolution he was mobilized into the Red Army, where he fought against General Denikin and served in the Russian Telegraph Agency.
Split into simpler sentences: He did not finish the "gimnaziya" but volunteered for the army in 1915, serving in the artillery. After the October Revolution he was mobilized into the Red Army, where he fought against General Denikin and served in the Russian Telegraph Agency.