The German attack has started in April 1941

 The Germans hastily regrouped after they heard of the Yugoslav coup. The armored group commanded by Kleist was to attack north from Sofia in the direction of Belgrade. This was an armored group in name only, as it had but one armored division. It also had one mountain, one motorized and two infantry divisions. A corp would move into "southern Yugoslavia", hoping to break up a Yugoslav army and join up with Italians. Another corps would push through the "Metaxas line' The plan was operating under the assumption that there some sixteen Yugoslav divisions located near Nish, Skoplje, and Velos. 

The primitive road system caused problems for the Germans. "The roads were narrow and winding". Some of the German guns were so long that they could not be towed on the road. German engineers resorted to explosives to make it possible for the guns to move. Another issue was that in Yugoslavia, there were rivers affected by the spring thaw. 

Troops on the right for the Germans had been able to cross "the Mur and Drave rivers". What was helpful was that they were able to capture some bridges that were still intact. The Germans took many Yugoslav prisoners as they advanced. Another factor that aircraft from the Yugoslav air force flew to Austria to surrender. Another issue was that very few German troops were actually in the field. The German commander decided that they would follow the original plan. That meant that they would have to wait until 10 April to move on Belgrade. In Yugoslavia, "the people were shaken by the treachery of the Regent, Prince Paul. Some mililtary leaders were also involved. That meant that there was less resistance to an attack, because of knowing about the political situation. The Germans took Vardar on 6 April, Skoplje was taken on 7 April. By this point, the Yugoslav army was mostly not fighting. This is based on the account in "Greece Crete and Syria" by Gavin Long.