Complex sentence: A sample set; (a) (east) The Third Army was concerned of the possibility of armed insurrection in Van Vilayet that they shifted forces to meet the threat, leaving Russian forces unchecked.
Split into simpler sentences: A sample set; (a) In the east, the Third Army was concerned about the possibility of armed insurrection in the Van Vilayet. So they shifted forces to meet the threat, leaving Russian forces unchecked.