Scholars have proposed several theories for the Royal Yugoslav Army's sudden collapse, including poor training and equipment, generals eager to secure a quick cessation of hostilities, and a sizeable Croatian nationalist fifth column. The invasion ended when an armistice was signed on 17 April 1941, based on the unconditional surrender of the Yugoslav army, which came into effect at noon on 18 April. Yugoslavia was then occupied and partitioned by the Axis powers. Some areas of Yugoslavia were annexed by neighboring Axis countries, some areas remained occupied, and in other areas Axis puppet states such as the Independent State of Croatia (, or NDH) were created during the invasion on 10 April. Along with Italy's stalled invasion of Greece on 28 October 1940, and the German-led invasion of Greece (Operation "Marita") and invasion of Crete (Operation "Merkur"), the invasion of Yugoslavia was part of the German Balkan Campaign ().