Complex sentence: During World War I the 10th army was stationed at the Eastern Front against Russia, and occupied Poland and Belorussia at the end of 1918 when the war ended.
Split into simpler sentences: During World War I the 10th Army was stationed on the Eastern Front where it fought against Russia. It also took part in the occupation of Poland and Belorussia at the end of 1918 when the war ended.