Complex sentence: Kapp was politically active during the First World War: in the early summer of 1916 he produced a secret pamphlet critical of German foreign and domestic policy under Chancellor Bethmann-Hollweg.
Split into simpler sentences: Kapp was politically active during the First World War. In the early summer of 1916 he produced a secret pamphlet critical of German foreign and domestic policy under Chancellor Bethmann-Hollweg.