Short answers, pulled from the story.
The Invasion of Yugoslavia began on the 6th of April 1941 following a coup d'état executed by Yugoslav military officers in Belgrade on the 27th of March 1941. Adolf Hitler issued Führer Directive No. 25 later that same day ordering the destruction of Yugoslavia as a state entity after learning of the change in government.
German forces prepared their attack from bases located in Romania, Bulgaria, and Hungary. Romanian oil fields at Ploiești remained vital to the Nazi war effort while the Luftwaffe placed 355 aircraft in Bulgaria by March 1941.
The Royal Yugoslav Navy possessed three destroyers, two submarines, and ten motor torpedo boats as its most effective units at the start of the invasion. Early morning attacks by German Ju 87 dive-bombers struck river monitors on the 12th of April leading to the scuttling of remaining vessels when enemy forces occupied bases and river systems.
An armistice signed on the 17th of April came into effect at noon on the 18th of April ending eleven days of fighting. The Kingdom of Yugoslavia was subsequently occupied and partitioned among Axis powers including Germany, Hungary, Italy, Albania, and Bulgaria.
Scholars argue that the main reason for collapse was that subordinate national groups including Slovenes and Croats refused to fight for a Serbian-dominated Yugoslavia. Defenses were built on the northern border when natural lines lay further south along rivers Sava and Drina.