The 1st Infantry Division (Mechanized) of the United States Army —nicknamed “The Big Red One” after its shoulder patch—is the oldest continuously serving division in the United States Army.

The British 1st Division was first formed in 1809 when the Duke of Wellington formed it, in Portugal, from two British brigades and one Hanoverian brigade of the King's German Legion.

The South African 1st Infantry Division was an infantry division of the South African Army during World War II.

The German 1st Infantry Division, (designated 1.Infanterie-Division in German), was one of the original infantry divisions of the Reichswehr and Wehrmacht and served throughout World War II.

The Slovak 1st Infantry Division was an infantry unit in the Slovak Army during World War II.