Balkans campaign (World War II) in the context of "Allies in World War II"

World War II or the Second World War (1 September 1939 – 2 September 1945) was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising all resources in pursuit of total war. Tanks and aircraft played major roles, enabling the strategic bombing of cities and delivery of the first and only nuclear weapons ever used in war. World War II is the deadliest conflict in history, causing the death of 70 to 85 million people, more than half of whom were civilians. Millions died in genocides, including the Holocaust, and by massacres, starvation, and disease. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were put on trial for war crimes.

The causes of World War II included unresolved tensions in the aftermath of World War I, the rise of fascism in Europe and militarism in Japan. Key events preceding the war included Japan's invasion of Manchuria in 1931, the Spanish Civil War, the outbreak of the Second Sino-Japanese War in 1937, and Germany's annexations of Austria and the Sudetenland. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland, after which the United Kingdom and France declared war on Germany. Poland was divided between Germany and the Soviet Union under the Molotov–Ribbentrop Pact. In 1940, the Soviet Union annexed the Baltic states and parts of Finland and Romania. After the fall of France in June 1940, the war continued mainly between Germany, now assisted by Fascist Italy, and the British Empire, with fighting in the Balkans, Mediterranean, and Middle East, the aerial Battle of Britain and the Blitz, and the naval Battle of the Atlantic. Through campaigns and treaties, Germany gained control of much of continental Europe and formed the Axis alliance with Italy, Japan, and other countries. In June 1941, Germany invaded the Soviet Union, opening the Eastern Front and initially making large territorial gains.

The Allies, formally referred to as the United Nations from 1942, were an international military coalition formed during World War II (1939–1945) to oppose the Axis powers. Its principal members were the "Big Four" – the United Kingdom, Soviet Union, United States, and China.

Membership in the Allies varied during the course of the war. When the conflict broke out on 1 September 1939, the Allied coalition consisted of the United Kingdom, France, and Poland, as well as their respective dependencies, such as British India. They were joined by the independent dominions of the British Commonwealth: Canada, Australia, New Zealand and South Africa. Consequently, the initial alliance resembled that of the First World War. As Axis forces began invading northern Europe and the Balkans, the Allies added the Netherlands, Belgium, Norway, Greece, and Yugoslavia. The Soviet Union, which initially had a nonaggression pact with Germany and participated in its invasion of Poland, joined the Allies after the German invasion of the Soviet Union in June 1941. The United States, while providing some material support to European Allies since September 1940, remained formally neutral until the Japanese attack on Pearl Harbor in December 1941, after which it declared war and officially joined the Allies. China had already been at war with Japan since 1937, and formally joined the Allies in December 1941.

The Greco-Italian War, also called the Italo-Greek War, took place between Italy and Greece from 28 October 1940 to 23 April 1941. This conflict began the Balkans campaign of World War II between the Axis powers and the Allies, and eventually turned into the Battle of Greece with British and German involvement. On 10 June 1940, Italy declared war on France and the United Kingdom. By September 1940, the Italians had invaded France, British Somaliland and Egypt. This was followed by a hostile press campaign in Italy against Greece, accused of being a British ally. A number of provocations culminated in the sinking of the Greek light cruiser Elli by the Italians on 15 August. On 28 October, Mussolini issued an ultimatum to Greece demanding the cession of Greek territory, which the Prime Minister of Greece, Ioannis Metaxas, rejected.

Italy's invasion of Greece, launched with the divisions of the Royal Army based in Italian-controlled Albania, badly armed and poorly commanded, resulted in a setback: the Italian forces encountered unexpectedly tenacious resistance by the Hellenic Army and penetrated only a few kilometers into Greek territory and had to contend with the mountainous and muddy terrain on the Albanian–Greek border. With British air and material support, the Greeks stopped the Italian invasion just inside Greek territory by mid-November and subsequently counter-attacked with the bulk of their mobilized army to push the Italians back into Albania – an advance which culminated in the Capture of Klisura Pass in January 1941, a few dozen kilometers inside the Albanian border. The defeat of the Italian invasion and the Greek counter-offensive of 1940 have been called the "first Axis setback of the entire war" by Mark Mazower.

The German invasion of Greece or Operation Marita (German: Unternehmen Marita), were the attacks on Greece by Italy and Germany during World War II. The Italian invasion in October 1940, which is usually known as the Greco-Italian War, was followed by the German invasion in April 1941. German landings on the island of Crete (May 1941) came after Allied forces had been defeated in mainland Greece. These battles were part of the greater Balkans Campaign of the Axis powers and their associates.

Following the Italian invasion on 28 October 1940, Greece, with British air and material support, repelled the initial Italian attack and a counter-attack in March 1941. When the German invasion, known as Operation Marita, began on 6 April, the bulk of the Greek Army was on the Greek border with Albania, then a vassal of Italy, from which the Italian troops had attacked. German troops invaded from Bulgaria, creating a second front. Greece received a small reinforcement from British, Australian and New Zealand forces in anticipation of the German attack. The Greek army found itself outnumbered in its effort to defend against both Italian and German troops. As a result, the Metaxas defensive line did not receive adequate troop reinforcements and was quickly overrun by the Germans, who then outflanked the Greek forces at the Albanian border, forcing their surrender. British, Australian and New Zealand forces were overwhelmed and forced to retreat, with the ultimate goal of evacuation. For several days, Allied troops played an important part in containing the German advance on the Thermopylae position, allowing ships to be prepared to evacuate the units defending Greece. The German Army reached the capital, Athens, on 27 April and Greece's southern shore on 30 April, capturing 7,000 British, Australian and New Zealand personnel and ending the battle with a decisive victory. The conquest of Greece was completed with the capture of Crete a month later. Following its fall, Greece was occupied by the military forces of Germany, Italy and Bulgaria.

The Royal Italian Army (Italian: Regio Esercito, lit. 'Royal Army', RE) was the land force of the Kingdom of Italy, established with the proclamation of the Kingdom of Italy. During the 19th century Italy started to unify into one country, and in 1861 Manfredo Fanti signed a decree creating the Army of the Two Sicilies. This newly created army's first task was to defend its territorial gains against Legitimists in southern Italy, who remained loyal to Francis II of the Two Sicilies. The Army of the Two Sicilies also waged what many modern historians now consider a civil war against outlaws and Bourbonist guerrillas, such as the famous Michelina Di Cesare, and against other Italian states' armies during the continuing wars of unification. After the monarchy ended in 1946, the army changed its name to become the modern Italian Army (Esercito Italiano).

Within the Royal Italian Army were the elite mountain military corporals, the Alpini. The Alpini, which remain in existence today, are the oldest active mountain infantry in the world. Their original mission was to protect and secure Italy's northern mountain border that aligns with France and Austria. This group emerged in World War I when a three-year campaign was fought against the Austro-Hungarian Kaiserjäger and the German Alpenkorps. In World War II, the Alpini fought alongside Axis forces in the Eastern Front as well as the Balkans campaign.