The current numbering system for army divisions was devised when the U.S. entered World War I.
Regular army divisions (those created from regiments of the standing army) were numbered 1 - 20; divisions formed from National Guard regiments were numbered 26 - 42; and divisions formed from drafted men (what was termed the National Army) were numbered 76 - 102. Some of these divisions were only partially formed, or formed on paper only and never activated.
Over the years, reorganizations take place within the army. Divisions and subordinate units are deactivated or amalgamated with other units. When this occurs, the division number (or other unit number) disappears from the active army table of organization. It may reappear later if the unit is reactivated or it may disappear forever, depending upon the needs of the army.
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