"Historical Armenia's boundaries have varied considerably, but old Armenia extended over what is now the northeastern part of Turkey and the Republic of Armenia." (Encyclopedia Britannica)Descriptive usage tells us that Armenia (Republic) is used for the period after 1991, and for the revolutionary government from 1918-1920, while Armenia refers only to the pre-1918 kingdom.
But Subject usage tells us that the heading Armenia is used for the time period until 1920.
Thus, for the revolutionary government of 1918-1920, there is the confusing situation that the name heading is Armenia (Republic), but subject usage is Armenia.
Subject Usage: This heading is used for works on Armenia since 1920. Works on the revolutionary events in the historical region of Armenia, including the Armenian Republic, 1918-1920, are entered under the subject heading Armenia--History--Revolution, 1917-1920.