Scholars have proposed several theories to explain the sudden collapse of the Royal Yugoslav forces, including poor training and equipment, generals eager to secure a quick cessation of hostilities, and fifth column activities by groups of Croatians, Slovenians, and ethnic Germans.

Most of Serbia and the Banat became a German zone of occupation while other areas of Yugoslavia were annexed by neighboring Axis countries, Germany, Hungary, Italy, Albania and Bulgaria.

[20] Hitler took the coup as a personal insult, and was so angered that he was determined, in his words, "to destroy Yugoslavia militarily and as a state" (Jugoslawien militärisch und als Staatsgebilde zu zerschlagen)[21] and to do so "with pitiless harshness"[22] and "without waiting for possible declarations of loyalty of the new government".

[30][31] After the Italian invasion in the northwest began, King Victor Emmanuel III moved to a villa owned by the Pirzio Biroli family at Brazzacco, near Moruzzo, in order to be close to the front.

The Wehrmacht entered Bulgaria more circumspectly, first with the intention of providing aerial defense against any force attacking Romania's oilfields and later with that of invading Greece in support of Italy.

[38] By the middle of November, the 13th Motorised Infantry Division had been assembled in Romania and reinforced by the 4th Panzer Regiment, engineers and signal troops, as well as six fighter and two reconnaissance Luftwaffe squadrons, and some antiaircraft artillery.

[43] Shortly thereafter a secret German team under Colonel Kurt Zeitzler entered Bulgaria to establish fuel depots, arrange for troop billeting and scout the terrain.

On 24 December, István Horthy, President of Hungarian State Railways (HSR), demanded negotiations before implementing requested German increases, but Ambassador Otto von Erdmannsdorf informed him that it had all been settled in Vienna by Keitel and Csáky.

[9] The VVKJ had a strength of 1,875 officers and 29,527 other ranks,[69] including some 2,000 pilots,[5] had over 460 front-line aircraft of domestic (notably the IK-3), German, Italian, French, and British origin, of which most were modern types.

The Naval Aviation units comprised 75 aircraft in eight squadrons equipped with, amongst other auxiliary types, 12 German-built Dornier Do 22 K and 15 Rogozarski SIM-XIV-H locally designed and built maritime patrol float-planes.

[71] The aircraft of the Yugoslav airline Aeroput, consisting mainly of six Lockheed Model 10 Electras, three Spartan Cruisers, and one de Havilland Dragon were mobilised to provide transport services to the VVKJ.

In its worst expression, Yugoslavia's defenses were badly compromised on 10 April 1941, when some of the units in the Croatian-manned 4th and 7th Armies mutinied,[77] and a newly formed Croatian government hailed the entry of the Germans into Zagreb the same day.

[91] The bomber and maritime force hit targets in Italy, Germany (Austria), Hungary, Romania, Bulgaria, Albania and Greece, as well as attacking German, Italian and Hungarian troops.

During this operation, Yugoslav gold reserves were also airlifted to Greece by the seven Do 17s,[96] as well as by SM-79Ks and Lockheed Electra's but after completing their mission, five Do 17Ks were destroyed on the ground when Italian aircraft attacked the Greek-held Paramitia airfield.

[72] At 16:00 on 15 April the C-in-C of Luftflotte 4, Generaloberst Alexander Löhr received orders from Hermann Göring to wind down the air-offensive and transfer the bulk of the dive-bomber force to support the campaign in Greece.

[99] Flying in relays from airfields in Austria and Romania, 300 aircraft, of which a quarter were Junkers Ju 87 Stukas, protected by a heavy fighter escort began the attack.

[101] Having thus delivered the knockout blow to the Yugoslavian nerve center, the Luftwaffe was able to devote its maximum effort to military targets such as Yugoslav airfields, routes of communication, and troop concentrations, and to the close support of German ground operations.

[109] A scheduled Yugoslav attack against the Italian enclave of Zara did not materialize, and the city garrison's troops started to advance until they met the "Torino" Division near Knin, which was taken on the same day.

[110] After repelling the Yugoslav offensive in Albania, the 18th Infantry Division Messina took Cetinje, Dubrovnik and Kotor on 17 April, meeting with the Italian units of the Motorized Corps.

In accordance with the Yugoslav Army's war plan, R-41, a strategy was formulated that, in the face of a massive Axis attack, a retreat on all fronts except in the south be performed.

[113] On 8 April the hard-pressed VVKJ sent a squadron of fourteen Breguet 19 light bombers to the city of Florina in northern Greece to provide assistance to both the Yugoslav and Greek Armies on the Macedonian front.

[114] The first elements of the 3rd Army launched their offensive operations in North Albania on 7 April 1941, with the Komski Odred covering the Gusinje-Prokletije mountains area advancing towards the village of Raja-Puka.

Vladko Maček and ban Ivan Šubašić sent messages to the city urging the regiments to maintain their positions, but this was disobeyed by the rebel military and civil officials who waited for the arrival of the German army.

[126] The naval prong to this attack faltered when the destroyer Beograd was damaged by near misses from Italian aircraft off Šibenik when her starboard engine was put out of action, after which she limped to Kotor, escorted by the remainder of the force, for repair.

On 9 April, one Dornier Do 22K floatplane notably took on an Italian convoy of 12 steamers with an escort of eight destroyers crossing the Adriatic during the day, attacking single-handed in the face of intense AA fire.

Several dozen of the escapee aircraft were destroyed in a devastating strafing attack by the Italian air force on Paramitia airfield in Greece, with nine bombers and transports making it to Egypt.

[127] However, in defiance of the said provisions, one destroyer, the Zagreb, was blown up at Kotor by two of its junior officers and one of the British-built submarines and two MTBs escaped to Alexandria in Egypt to continue to serve with the Allied cause.

[144] On 16 April, a Yugoslav delegate arrived in Belgrade, but as he did not have authority to sign the document, he was given a draft of the agreement, and an aircraft was placed at his disposal to bring in authorised representatives of the government.

Finally, on 17 April, after only eleven days of fighting, the pre-coup Foreign Minister Aleksandar Cincar-Marković and General Radivoje Janković signed the armistice and unconditionally surrendered all Yugoslav troops.

[146] Some scholars have proposed a number of 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, Slovenian and German fifth column.