Along with the rest of the Royal Yugoslav Army, the 1st Cavalry Division began mobilising on following a coup d'état. Three days later, with mobilisation not complete, the Germans began an air campaign and a series of preliminary operations against the Yugoslav frontiers. By the end of the following day, the division's cavalry brigade headquarters and all of the division's cavalry regiments had been detached for duty with other formations of the 1st Army Group. The divisional headquarters and divisional-level units remained in the vicinity of Zagreb until , when they were given orders to establish a defensive line southeast of Zagreb along the Sava River, with infantry and artillery support. The division had only begun to deploy for this task when the German 14th Panzer Division captured Zagreb. The divisional headquarters and all attached units were then captured by armed Croat fifth column groups, or surrendered to German troops.