'Ost' (iii) was a German undertaking under the leadership of Generalmajor Erich Marcks, chief-of-staff of Generaloberst Georg von Küchler’s 18^th Army, to reconnoitre and fix the demarcation line in Poland between the German and Soviet forces in the aftermath of 'Weiss' (i) (5 August/September 1940).

By this time German thought and planning were concentrated on the invasion of the USSR planned for the spring of 1941, and the 'Ost' (iii) effort was therefore important, from the German perspective, inasmuch as it finalised the start lines for the forthcoming 'Barbarossa'.