'Eisenes Tor' was the German contribution to 'Unternehmen 25' and 'Marita' by Generalfeldmarschall Wilhelm List’s 12^th Army, which advanced from the east into south-western Yugoslavia from eastern Bulgaria (6/30 April 1941).

In this undertaking, Generaloberst Ewald von Kleist’s Panzergruppe 'von Kleist' (General Joachim von Kortzfleisch’s XI Corps and General Gustav von Wietersheim’s XIV Korps [mot.]) attacked to the west from points north of Sofia in Bulgaria to take Nis on the Morava river and then wheel to the north-west down this river toward Belgrade, General Georg Stumme’s XL Korps (mot.) attacked on two axes from points south-west of Sofia to take Skopje on the Vardar river and then push forward to Monastir on the Yugoslav side of the Yugoslav/Greek frontier, and farther to the south General Franz Böhme’s XVIII Corps advanced to the south-west to take Strumica and then wheel left toward Thessaloníki in northern Greece.