The continued resistance of the British caused Hitler once more to change his timetable. His great design for a campaign against the U.S.S.R. had originally been scheduled to begin about 1943—by which time he should have secured the German position on the rest of the European continent by a series of “localized” campaigns and have reached some sort of compromise with Great Britain. But in July 1940, seeing Great Britain still undefeated and the United States increasingly inimical to Germany, he decided that the conquest of the European part of the Soviet Union must be undertaken in May 1941 in order both to demonstrate Germany’s invincibility to Great Britain and to deter the United States from intervention in Europe (because the elimination of the U.S.S.R. would strengthen the Japanese position in the Far East and in the Pacific). Events in the interval, however, were to make him change his plan once again.

While the invasion of the U.S.S.R. was being prepared, Hitler was much concerned to extend German influence across Slovakia and Hungary into Romania, the oil fields of which he was anxious to secure against Soviet attack and the military manpower of which might be joined to the forces of the German coalition. In May 1940 he obtained an oil and arms pact from Romania; but, when Romania, after being constrained by a Soviet ultimatum in June to cede Bessarabia and northern Bukovina to the U.S.S.R., requested a German military mission and a German guarantee of its remaining frontiers, Hitler refused to comply until the claims of other states against Romania had been met. Romania was compelled to cede southern Dobruja to Bulgaria on August 21 (an act that was formalized in the Treaty of Craiova on September 7); but its negotiations with Hungary about Transylvania were broken off on August 23. Since, if war had broken out between Romania and Hungary, the U.S.S.R. might have intervened and won control over the oil wells, Hitler decided to arbitrate immediately: by the Vienna Award of August 30, Germany and Italy assigned northern Transylvania, including the Szekler district, to Hungary, and Germany then guaranteed what was left of Romania. In the face of the Romanian nationalists’ outcry against these proceedings, the king, Carol II, transferred his dictatorial powers to General Ion Antonescu on Sept. 4, 1940, and abdicated his crown in favour of his young son Michael two days later. Antonescu had already repeated the request for a German military mission, which arrived in Bucharest on October 12.

Though Hitler had apprised the Italian foreign minister, Galeazzo Ciano, of his intention to send a military mission to Romania, Ciano had not apprised Mussolini. So, since the latter’s Balkan ambitions had been continually restrained by Hitler, particularly with regard to Yugoslavia, the sudden news of the mission annoyed him. On Oct. 28, 1940, therefore, having given Hitler only the barest hints of his project, Mussolini launched seven Italian divisions (155,000 men) from Albania into a separate war of his own against Greece.

The result was exasperating for Hitler. His ally’s forces were not only halted by the Greeks, a few miles over the border, on Nov. 8, 1940, but were also driven back by General Alexandros Papagos’ counteroffensive of November 14, which was to put the Greeks in possession of one-third of Albania by mid-December. Moreover, British troops landed in Crete, and some British aircraft were sent to bases near Athens, whence they might have attacked the Romanian oil fields. Lastly, the success of the Greeks caused Yugoslavia and Bulgaria, who had hitherto been attentive to overtures from the Axis powers, to revert to a strictly neutral policy.

Anticipating Mussolini’s appeal for German help in his “separate” or “parallel” war, Hitler in November 1940 drew Hungary, Romania, and Slovakia successively into the Axis, or Tripartite, Pact that Germany, Italy, and Japan had concluded on September 27 (see below Japanese policy, 1939–41); and he also obtained Romania’s assent to the assembling of German troops in the south of Romania for an attack on Greece through Bulgaria. Hungary consented to the transit of these troops through its territory lest Romania take Hungary’s place in Germany’s favour and so be secured in possession of the Transylvanian lands left to it by the Vienna Award. Bulgaria, however, for fear of Soviet reaction, on the one hand, and of Turkish, on the other (Turkey had massed 28 divisions in Thrace when Italy attacked Greece), delayed its adhesion to the Axis until March 1, 1941. Only thereafter, on March 18, did the Yugoslav regent, Prince Paul, and his ministers Dragiša Cvetković and Aleksandar Cincar-Marković agree to Yugoslavia’s adhesion to the Axis.

Meanwhile, the German 12th Army had crossed the Danube from Romania into Bulgaria on March 2, 1941. Consequently, in accordance with a Greco-British agreement of February 21, a British expeditionary force of 58,000 men from Egypt landed in Greece on March 7, to occupy the Olympus–Vermion line. Then, on March 27, 1941, two days after the Yugoslav government’s signature, in Vienna, of its adhesion to the Axis Pact, a group of Yugoslav Army officers, led by General Dušan Simović, executed a coup d’état in Belgrade, overthrowing the regency in favour of the 17-year-old king Peter II and reversing the former government’s policy.

Almost simultaneously with the Belgrade coup d’état, the decisive Battle of Cape Matapan took place between the British and Italian fleets in the Mediterranean, off the Peloponnesian mainland northwest of Crete. Hitherto, Italo-British naval hostilities in the Mediterranean area since June 1940 had comprised only one noteworthy action: the sinking in November at the Italian naval base of Taranto of three battleships by aircraft from the British carrier Illustrious. In March 1941, however, some Italian naval forces, including the battleship Vittorio Veneto, with several cruisers and destroyers, set out to threaten British convoys to Greece; and British forces, including the battleships Warspite, Valiant, and Barham and the aircraft carrier Formidable, likewise with cruisers and destroyers, were sent to intercept them. When the forces met in the morning of March 28, off Cape Matapan, the Vittorio Veneto opened fire on the lighter British ships but was soon trying to escape from the engagement, for fear of the torpedo aircraft from the Formidable. The battle then became a pursuit, which lasted long into the night. Finally, though the severely damaged Vittorio Veneto made good her escape, the British sank three Italian cruisers and two destroyers. The Italian Navy made no more surface ventures into the eastern Mediterranean.

The German attack on Greece, scheduled for April 1, 1941, was postponed for a few days when Hitler, because of the Belgrade coup d’état, decided that Yugoslavia was to be destroyed at the same time. While Great Britain’s efforts to draw Yugoslavia into the Greco-British defensive system were fruitless, Germany began canvasing allies for its planned invasion of Yugoslavia and Greece. Italy agreed to collaborate in the attack, and Hungary and Bulgaria agreed to send troops to occupy the territories that they coveted as soon as the Germans should have destroyed the Yugoslav state.

On April 6, 1941, the Germans, with 24 divisions and 1,200 tanks, invaded both Yugoslavia (which had 32 divisions) and Greece (which had 15 divisions). The operations were conducted in the same way as Germany’s previous blitzkrieg campaigns. While massive air raids struck Belgrade, List’s 12th Army drove westward and southward from the Bulgarian frontiers, Kleist’s armoured group northwestward from Sofia, and Weichs’s 2nd Army southward from Austria and from western Hungary. The 12th Army’s advance through Skopje to the Albanian border cut communications between Yugoslavia and Greece in two days; Niš fell to Kleist on April 9, Zagreb to Weichs on April 10; and on April 11 the Italian 2nd Army (comprising 15 divisions) advanced from Istria into Dalmatia. After the fall of Belgrade to the German forces from bases in Romania (April 12), the remnant of the Yugoslav Army—whose only offensive, in northern Albania, had collapsed—was encircled in Bosnia. Its capitulation was signed, in Belgrade, on April17.

In Greece, meanwhile, the Germans took Salonika (Thessaloníki) on April 9, 1941, and then initiated a drive toward Ioánnina (Yannina), thus severing communication between the bulk of the Greek Army (which was on the Albanian frontier) and its rear. The isolated main body capitulated on April 20, the Greek Army as a whole on April 22. Two days later the pass of Thermopylae, defended by a British rear guard, was taken by the Germans, who entered Athens on April 27. All mainland Greece and all the Greek Aegean islands except Crete were under German occupation by May 11, the Ionian islands under Italian. The remainder of Britain’s 50,000-man force in Greece was hastily evacuated with great difficulty after leaving all of their tanks and other heavy equipment behind.

The campaign against Yugoslavia brought 340,000 soldiers of the Yugoslav Army into captivity as German prisoners of war. In the campaign against Greece the Germans took 220,000 Greek and 20,000 British or Commonwealth prisoners of war. The combined German losses in the Balkan campaigns were about 2,500 dead, 6,000 wounded, and 3,000 missing.

German airborne troops began to land in Crete on May 20, 1941, at Máleme, in the Canea-Suda area, at Réthimnon, and at Iráklion. Fighting, on land and on the sea, with heavy losses on both sides, went on for a week before the Allied commander in chief, General Bernard Cyril Freyberg of the New Zealand Expeditionary Force, was authorized to evacuate the island. The last defenders were overwhelmed at Réthimnon on May 31. The prisoners of war taken by the Germans in Crete numbered more than 15,000 British or Commonwealth troops, besides the Greeks taken. In battles around the island, German air attacks sank three light cruisers and six destroyers of the British Mediterranean fleet and damaged three battleships, one aircraft carrier, six light cruisers, and five destroyers.

Both the Yugoslav and the Greek royal governments went into exile on their armies’ collapse. The Axis powers were left to dispose as they would of their conquests. Yugoslavia was completely dissolved: Croatia, the independence of which had been proclaimed on April 10, 1941, was expanded to form Great Croatia, which included Srem (Syrmia, the zone between the Sava and the Danube south of the Drava confluence) and Bosnia and Hercegovina; most of Dalmatia was annexed to Italy; Montenegro was restored to independence; Yugoslav Macedonia was partitioned between Bulgaria and Albania; Slovenia was partitioned between Italy and Germany; the Baranya triangle and the Bačka went to Hungary; the Banat and Serbia were put under German military administration. Of the independent states, Great Croatia, ruled by Ante Pavelić’s nationalist Ustaše (“Insurgents”), and Montenegro were Italian spheres of influence, although German troops still occupied the eastern part of Great Croatia. A puppet government of Serbia was set up by the Germans in August 1941.

While Bulgarian troops occupied eastern Macedonia and most of western Thrace, the rest of mainland Greece, theoretically subject to a puppet government in Athens, was militarily occupied by the Italians except for three zones, namely the Athens district, the Salonika district, and the Dimotika strip of Thrace, which the German conquerors reserved for themselves. The Germans also remained in occupation of Lesbos, Chios, Samos, Melos, and Crete.