 The Yugoslav coup d 'état led to a sudden change in the plan of attack and confronted the Twelfth Army with a number of difficult problems . According to the 28 March Directive No. 25 , the Twelfth Army was to create a mobile task force to attack via Niš toward Belgrade . With only nine days left before their final deployment , every hour became valuable and each fresh assembly of troops took time to mobilise . By the evening of 5 April , the forces intended to enter southern Yugoslavia and Greece had been assembled . 
