Shortly before the invasion, Serbian officers of the Yugoslav General Staff, encouraged by British Special Operations Executive personnel in Belgrade, led a coup d'état against Prince Paul of Yugoslavia and Dragiša Cvetković for adhering to the Tripartite Pact.

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

A National Council of Slovenes, Croats and Serbs had been formed in Zagreb in the previous month with the aim of representing the kingdoms of Croatia-Slavonia and Dalmatia, the condominium of Bosnia and Herzegovina, and the Slavic-populated areas of Carniola and Styria.

[3] Immediately after the Armistice of Villa Giusti, Italy began occupying parts of the Kingdom of Dalmatia that had been promised to it under the secret Treaty of London.

[9] The Serbian Army numbered 145,225 soldiers at the end of the war, and absorbed the some 15,000 former Austro-Hungarian officers and volunteers which had been organized by the National Council.

[6] From late 1918 until 10 September 1919, the new army was involved in a sharp military confrontation with irregular pro-Austrian formations in the region of Carinthia on the northern frontier of the new KSCS.

[12] To deal with these security concerns, a large mobilization was carried out from 1918 to 1919, reaching a peak of 450,000 soldiers in July 1919, though demobilization quickly followed.

[15] In the early 1920s, the army responded to several external crises, including the attempted return of King Charles IV to neighbouring Hungary, disturbances along the Albanian border,[16] and incursions from Bulgaria.

One year after their disbandment, border disturbances made it necessary to reconstitute a smaller contingent of frontier troops in the 3rd Army area.

[21] The first significant acquisition of military aircraft were made in the same year, with 150 Breguet 19 light bomber and aerial reconnaissance biplanes being purchased from France under the terms of a loan.

In the same year, 13 more companies of frontier troops were raised for deployment along the Hungarian and Italian borders,[25] and 12 Dornier floatplanes were also purchased.

[34] Inter-division manoeuvres were again undertaken in three regions, but cavalry charges and massed infantry attacks demonstrated that the army had not learned the lessons of World War I.

This latter development was intended as the first step to creating two independent formations that, with integral artillery, signals and transport elements, could be used along the mountainous northwest frontier.

[39] Some communist activity was detected within the army during the year, and the same conservative group of senior Serb officers remained firmly in charge.

From a military perspective, it was intended that the Chetniks would assist the frontier guards in peacetime, in addition to their traditional guerilla activities in times of war.

Equipment received during the year included 800 Stokes mortars, enough Skoda anti-aircraft guns to arm 20 batteries, and six Škoda Š-I-D tankettes.

The British military attaché observed that even the most senior commanders have never handled a force larger than a division on exercises or in war.

They took place on the Sava river between Novi Sad and Sarajevo at the end of September, and were really in the form of a demonstration rather than a war game.

Before this occurred Marić had told the British naval and military attachés that any mobilisation of the army would take 25 days, and revealed that shortfalls in many items of equipment were severe, including gas masks, steel helmets, tents, horseshoes, small arms ammunition, saddlery and tanks.

[49] Large-scale manoeuvres were carried out in Slovenia in September 1937, involving the equivalent of four divisions, and exposing to foreign observers the serious deficiencies in the army, caused by incompetent General Staff and the senior commanders, a lack of technical training of regimental officers in modern warfare, and across-the-board shortages of arms and equipment of almost every type.

The British military attaché observed that the army was not capable of undertaking any large-scale operations outside of the country, but if fully mobilised would be able to give a good account of itself in a defensive campaign.

[51] During 1938, Milutin Nedić was appointed as Minister of the Army and Navy, and was replaced as Chief of the General Staff by Armijski đeneral Dušan Simović.

That year, two geo-strategic changes made the task of the army significantly more difficult, the Anschluss between Germany and Austria, and the Munich Agreement which drastically weakened Czechoslovakia.

[52] It was the assessment of the British military attaché that the army could stem the tide of an invasion by one of its neighbours acting alone, with the possible exception of Germany, and could also deal with a combined Italian and Hungarian attack.

[53] During the year, a Coastal Defence Command was raised using troops already stationed along the Yugoslav coastline, and did not involve the creation of new formations.

The German attack, however, caught the army still mobilizing, and only some eleven divisions were in their planned defense positions at the start of the invasion.

The total strength of the Royal Yugoslav Army at full mobilization was about 1,200,000 however only around 50 per cent of the recruits were able to join their units before the German invasion.

The Coastal Defence Force, on the Adriatic opposite Zadar comprised one infantry division and two detachments, in addition to fortress brigades and anti-aircraft units at Šibenik and Kotor.

In November 1943, the Yugoslav Detachment was established as part of the 512th Bombardment Squadron of the United States Army Air Forces.

[65] All Royal Yugoslav Forces were formally disbanded on March 7, 1945, when King Peter II's government was abolished in Yugoslavia.