Complex sentence: By 1916, most of Western Armenia was occupied by Russian Empire as part of the Caucasian Campaign of World War I. In the occupied areas the Republic of Van was established until 1918, when the Russian army left the region due to the Revolution of 1917.
Split into simpler sentences: By 1916, most of Western Armenia was occupied by Russian Empire as part of the Caucasian Campaign of World War I. In parts of occupied areas, especially around Van, an Armenian autonomy was briefly set up. The Russian army left the region due to the Revolution of 1917.