Complex sentence: In the 1920s and much of the 1930s the General Staff tried to establish a small mechanized professional army, using the Experimental Mechanized Force as a prototype, but with the lack of any identified threat the Army's main function was to garrison the British Empire.
Split into simpler sentences: In the 1920s, and much of the 1930s, the General Staff tried to establish a small mechanized professional army, using the Experimental Mechanized Force as a prototype. However, with the lack of any identified threat, the Army's main function was to garrison the British Empire.