People of Belgrade demonstrate their support for break-up with the Tripartite Pact. "Rather death than slavery" was their slogan.

Consolidation of the influence of the hitlerite Germany in Hungary, Romania and Bulgaria went together with mounting pressure on Yugoslavia. For Germany Yugoslavia constituted a very important element of security of the German rear during operation Marita, as well as operation Barbaroßa. Adolf Hitler, according to his strategic policy in the Balkans, was seeking a "peaceful" solution. He did not nurse any illusions as to the real feelings of the peoples of Yugoslavia about the "Axis" Rome - Berlin, but he considered the position of the fascist bloc so unshakable, and its final victory so unquestionable, that the government of Yugoslavia, under appropriate pressure and lured with the perspective of obtaining Greek Macedonia with the port of Salonika, would rather turn its country into Germany's satellite than dare to fight the mighty III Reich.

Diplomatic advances towards Yugoslavia began yet in the autumn 1940. However for a while both prince Paul and the prime-minister Dragiša Cvetković evaded closer association with the III Reich. They suggested conclusion of a non-aggression treaty instead. But it could not satisfy Hitler, who on 24 February 1941 invited prince Paul to visit him. Simultaneously, on 2 March, German troops marched in to Bulgaria, which made Yugoslavia's encirclement almost complete. In those circumstances prince Paul secretly made for Bavaria, where he met Hitler on 4 March. There the Führer warmly invited his guest to join the Tripartite Pact and have his share in the "new world order", namely Salonika. Prince Paul though evaded a clear answer, and from Hitler's careless talking he learnt that Germany was soon going to invade the Soviet Union. The same was reported by the Yugoslav intelligence.

Upon return to Belgrade, prince Paul referred Hitler's plans to Cvetković, as well as his brother-in-law - the king of Greece, who in his turn informed the British. Both the regent and his prime-minister decided to procrastinate, but the III Reich did not left them room for manoeuvre. It was an actual ultimatum. Eventually, to avoid a cabinet crisis, prime-minister Cvetković signed on 25 March 1941 in Vienna the act of Yugoslavia's access to the Tripartite Pact. Apart of the official act, agreeing parties had signed a secret protocol, in which Germany and Italy promised to respect Yugoslav interests in Salonika, and not to demand a military support or transit rights for their troops across Yugoslavia.

The news about the Viennese agreement cause an uproar in Belgrade. Politicians, who sympathized with the West, began preparations to leave the country, a group of opposition politicians from Serbian parties started forming the Motherland Defence Front, and the first demonstrators took to the streets. Those developments were exploited by a group of officers rallied around the commander-in-chief of the Yugoslav air force, Gen. Dušan Simović. The actual leader of the group was Gen. Borivoje Mirković. The group represented patriotic circles within the officers' corps and opposition Serbian parties. Simović already during the critical days of making decisions to join the Tripartite Pact warned prince Paul that such a step would be pregnant with grave consequences. When it eventually happened, the group of conspirators decided to act. It was decided that at the night to 27 March the troops under the command of respective members of the plot would seize the most important places in Belgrade. Simultaneously the conspirators would pronounce king Peter II legally adult, dismiss the regency and the government, and form a new government with Simović in van.

According to the plan, the troops, led by the conspirators, seized designated objects, including the buildings of the council of ministers, central post office, and the General Staff, where at 3:00 arrived Simović. An hour later the radio-station of Belgrade transmitted on short waves the first broadcast that read:
On 27 March 1941 His Majesty King Peter II, having support of the armed forces, decided to take the power in his hands. The Regents and the Government have been dismissed. The mission to create a new Government has been entrusted to General Simovic. The law and order are maintained in the whole country. [Босанац М. (1964).]
Meanwhile the chief of the General Staff, Gen. Petar Kosić, who simultaneously was also the royal tutor and lived in the royal palace, as soon as he learnt about the events in the city and that all that had been done by the royal order, ran with the news to the king. The abruptly awakened Peter II only said that he knew nothing and that he barely knew Simović. The king summoned ad hoc a council with Kosić and the the commander of the royal guards, Gen. Mihailo Stajić. Next, all three went to the barracks of the royal guards, where the king inspected the troops. Gen. Kosić hoped that once the troops saw the king they would know on what side the monarch was. He also sent a message to Simović with the order to report at the royal court. In return Simović issued an order to apprehend Kosić and Stajić. The order had to be carried by a group of loyal officers reinforced by a tank battalion.

Meanwhile Kosić tried to mount a counter-action with the troops being readied in the marshalling yard of the royal guards' barracks. First he summoned the commander of a squadron of the horse artillery, Major Borot, and ordered him to assume fire positions. Borot, however, refused to carry the order. Then Kosić turned to the commander of the 1st Battery of the squadron, Capt. Ljubiša Džorđević, and passed the order to him, but Džorđević refused to obey too. Also commanders of other units refused to obey orders. Eventually they overpowered Kosić and escorted him to the building of the General Staff, where were already gathered members of the former government and two regents.

But the third and the most important regent, prince Paul, was missing. The day before he left for Slovenia for a short rest. The news about the coup d'état reached him on the railway station in Zagreb. He immediately summoned a conference with the head Croatian politicians, Dr. Vlatko Maček and ban Ivan Šubašić. They advised that the prince send to Simović an ultimatum demanding his immediate resignation, and in case if Simović refused, march on Belgrade in van of loyal regiments. But the prince, scared of the vision of a civil war, decided to return to Belgrade on his own. As soon as he arrived in the capital in the evening, he induced Peter II to seek a compromise with Simović, and he also handed his resignation over to the rebellious General. It was not until then that the king received Simović and formally entrusted him the mission of creating a new government. The compromise with Peter II and Simović meant accepting everything that the General had done in name of the king and eventually closed the final stage of the coup d'état. Late in the evening, still in the building of the General Staff, General Simović presided over the first official meeting of his cabinet.

Such a development of the events was possible to a big degree thanks to the support of the Yugoslav people, especially in Serbia. Already in the morning on 27 March, first in Belgrade and then in other cities, people took to the streets. They rose the slogans "Rather war than the pact", "Rather death than slavery", "We demand a pact with the Soviet Union, not with Germany", etc. In Slovenia similar demonstrations took place in the afternoon. In Zagreb, where the demonstrations were led by the members of the Communist Party of Yugoslavia, took place clashes with the Croat fascists, disappointed with the changes in Belgrade. Nationalist circles in Croatia were pleased with the conclusion of the pact with Germany, since they saw in it a chance to crush the state of the South Slavs and create a puppet political formation under the protectorate of the Nazi Reich.

The new government, formed by Gen. Simović, was in an exceptionally difficult position. Inside Yugoslavia it strove to find an agreement with the Croatian opposition, and even announced a partial amnesty. In the international affairs it sought a rapprochement with the Soviet Union. The agreement of mutual assistance was signed in Moscow at night from 5 to 6 April. There were also conducted talks with the British and Greeks, while the United States declared their material support. The latter was indeed very important, since Yugoslavia needed to mobilize her armed forces, which lacked modern equipment and supplies.

Of course the Yugoslavs tried to maintain good relations with Germany. Several hours after the coup d'état, in the morning of 27 March, the new Yugoslav foreign minister, Momčilo Nincić, assured the German envoy in Belgrade that Yugoslavia would continue a friendly policy towards Germany, and on 5 April he proposed direct talks with the German government. The talks, however, never happened. When news about the coup d'état in Belgrade reached Berlin, Hitler reacted with a paroxysm of convulsive rage. Later he reportedly said about the Yugoslav coup d'état that it came suddenly out of the blue. When the news was brought to me, on the morning of March 27, I thought it was a joke. In the evening of 27 March Hitler summoned a conference in the Reich Chancellery; present were Reichsmarschall Hermann Göring, Field-Marshal Wilhelm Keitel, General Alfred Jodl, and the foreign minister Joachim Ribbentrop. The Führer announced that the position of Yugoslavia was uneven from the point of view of the planned invasion both on Greece and on the Soviet Union. Therefore he ordered to destroy Yugoslavia militarily and as a nation before turning on the USSR, and instructed Ribbentrop to advise Hungary, Bulgaria and Italy that they would all get a slice of Yugoslavia if they took part in the attack on that country. The war against Yugoslavia should be very popular in Italy, Hungary and Bulgaria, Hitler sneered.

During that conference was laid the general plan of the war on Yugoslavia: it had to be swift, crushing and merciless. In new circumstances the operation Barbaroßa had to be postponed up to four weeks. Yet to make the onslaught on the USSR possible in the new term, one needed to make short with Yugoslavia as soon as possible. At the end of the conference the Directive No.25 was ready and Hitler signed it on the spot. The directive among others read:
It is my intention to break into Yugoslavia in the general direction of Belgrade and to the south by a concentric operation from the Fiume-Graz area on the one side, and the Sofia area on the other, and to deal an annihilating blow to the Yugoslav forces. Further, the extreme southern region of Yugoslavia will be cut off from the rest of the country and will be occupied as a base from which the German-Italian offensive against Greece can be continued. (...)

I issue the following detailed orders:
  1. As soon as sufficient forces are available and the weather allows, the ground installations of the Yugoslav Air Force and the city of Belgrade will be destroyed from the air by continual day and night attack.
  2. If possible simultaneously - but in no event earlier - "Undertaking Marita" will begin, with the temporarily limited objective of occupying Salonika basin and gaining foothold on the heights of Edessa.
Soon later Hitler sent to Benito Mussolini a message, which among others read:

Duce, events force me to give you, Duce, by this quickest means, my estimation of the situation and the consequences which may result from it.

From the beginning, I have regarded Yugoslavia as a dangerous factor in the controversy with Greece. Considered from the purely military point of view, German intervention in the war in Thrace would not be at all justified, as long as the attitude of Yugoslavia remains ambiguous and she could threaten the left flank of the advancing columns, on our enormous front. (...)

I don't consider this situation as being catastrophic but nevertheless a difficult one, and we, on our part, must avoid any mistake if we do not want, in the end, to endanger our whole position. (...)

Now I would cordially request you, Duce, not to undertake any further operations in Albania in the course of the next few days. I consider it necessary that you should cover and screen the most important passes from Yugoslavia into Albania with all available force.

These measures should not be considered as designed for a long period of time but as auxiliary measures designed to prevent for at least fourteen days to three weeks a crisis arising.

I also consider it necessary, Duce, that you should reinforce your forces on the Italian-Yugoslav front with all available means and with utmost speed. (...)

Duce, should secrecy be observed as to these measures, then in case action on our part should become necessary, I have no doubt that we will both achieve a success no less than the success in Norway a year ago. This is my unshaken conviction.

Still on 27 March the Führer met ambassadors of Hungary and Bulgaria, and in connection to the events in Yugoslavia discussed with them the perspectives of their common with Germany military action against Belgrade. Romania had to cover that action from the side of the Soviet Union, so Romania was not directly involved in the case of Yugoslavia. Both Italians and Hungarians accepted the plan of invasion of Yugoslavia, while Bulgaria tried to evade direct commitments. Simultaneously with the diplomatic action also began military preparations.

While only the 12th Army and the VIII Air Corps were already concentrated for combat operations, other forces still had to be deployed along the Yugoslav frontiers, and so the aggression had to commence on different days. On 6 April the air force had to strike on Belgrade, and the left wing of the 12th Army had to strike in the direction of Skopje and Salonika. Then the 1st Armoured group had to strike on 8 April from Sofia to Belgrade, and on 12 April the 2nd Army had to strike in western Yugoslavia. By that moment the Italians and Hungarians had to be ready to join the German action. Hitler had engaged in the Balkans huge forces - 2 million men, including the Italian troops in Albania, grouped in 80 divisions, and supported by 2,000 aircraft.