@cstross People get way too excited about drones.
From 1860 to 1914, the decisive force is rifle regiments; you see things like the reconquest of Sudan where enemy forces are shot to pieces at ranges above 1000 m.
1915 and subsequent, the continuous front and concentration means anything seen gets ground up by artillery from beyond rifle range. It's a pure mass contest. (and "mechanised warfare" dilutes by making the continuous front move. 1939-1945 is an historical anomaly.)