Operation Achse in the context of "Royal Italian Army during World War II"

Italy entered World War II on 10 June 1940 by invading France, joining the German offensive already in progress. Italian dictator Benito Mussolini did so opportunistically as the Allied powers (chiefly France and the United Kingdom) seemed on the verge of collapse. The Italian war aim was to expand its colonial empire at the expense of the French and the British. While France surrendered on 22 June 1940, the United Kingdom and its allies continued to fight far beyond the point which Mussolini had thought possible, ultimately leading to the defeat and dissolution of Fascist Italy in 1943 when Mussolini was deposed in a bloodless coup d'état.

Italy's Axis partner, Nazi Germany, was ready for its defection and occupied central and northern Italy after the armistice of Cassibile in September 1943. After freeing Mussolini from captivity, the Germans set him up as the leader of a new puppet state in the north, the Italian Social Republic. This provoked Italian resistance against the German occupation and also a civil war between pro- and anti-fascist Italians. In the Allied-held south, the Kingdom of Italy officially became a co-belligerent of the Allies and declared war on Germany on 13 October 1943.

The Italian Social Republic (Italian: Repubblica Sociale Italiana, IPA: [reˈpubblika soˈtʃaːle itaˈljaːna]; RSI, known prior to December 1943 as the National Republican State of Italy (Italian: Stato Nazionale Repubblicano d'Italia; SNRI), but more popularly known as the Republic of Salò (Italian: Repubblica di Salò, IPA: [reˈpubblika di saˈlɔ]), was a German puppet state with limited diplomatic recognition that was created during the latter part of World War II. It was a collaborationist regime in German-occupied Italy, established after the German invasion of Italy in September 1943 and disbanded with the surrender of Axis troops in Italy in May 1945. The German occupation triggered widespread national resistance against it and the Italian Social Republic during the liberation of Italy and the Italian civil war.

The Italian Social Republic was the second and last incarnation of the Italian Fascist state, led by the Italian dictator Benito Mussolini and his reformed Republican Fascist Party. The newly founded state declared Rome its capital but the de facto capital was Salò (hence the colloquial name of the state), a small town on Lake Garda, near Brescia, where Mussolini and the Ministry of Foreign Affairs were headquartered. The Italian Social Republic nominally exercised sovereignty in Northern and Central Italy, but was largely dependent on German troops to maintain control.

The Italian campaign of World War II, also called the Liberation of Italy following the German occupation in September 1943, consisted of Allied and Axis operations in and around Italy, from 1943 to 1945. The joint Allied Forces Headquarters (AFHQ) was operationally responsible for all Allied land forces in the Mediterranean theatre and it planned and led the invasion of Sicily in July 1943, followed in September by the invasion of the Italian mainland and the campaign in Italy until the surrender of the Axis forces in Italy in May 1945.

The invasion of Sicily in July 1943 led to the collapse of the Fascist Italian regime and the fall of Mussolini, who was deposed and arrested by order of King Victor Emmanuel III on 25 July. The new government signed an armistice with the Allies on 8 September 1943. However, German forces soon invaded northern and central Italy, committing several atrocities against Italian civilians and army units who opposed the German occupation and started the Italian resistance movement. Mussolini, who was rescued by German paratroopers, established a collaborationist puppet state, the Italian Social Republic (RSI), to administer the German-occupied territory. On 13 October 1943, the Allies recognized Italy as a co-belligerent in the war against Germany. Thereafter, the Italian Co-Belligerent Army and the Italian partisans fought alongside the Allies against German troops and the collaborationist National Republican Army; an aspect of this period is the Italian civil war. In the summer of 1944, after the Axis defeats at Cassino and Anzio, central Italy, including Rome, was liberated. Northern Italy was liberated following the final spring offensive and the general insurrection of Italian partisans on 25 April 1945. Mussolini was captured by the Italian resistance and summarily executed by firing squad. The campaign ended when Army Group C surrendered unconditionally to the Allies on 2 May 1945, one week before the formal German Instrument of Surrender. Both sides committed war crimes during the conflict, and the independent states of San Marino and Vatican City surrounded by Italian territory also suffered damage.

The Kingdom of the South (Italian: Regno del Sud) was the short period of the Kingdom of Italy from 1943 to 1944, when Pietro Badoglio controlled the country in the aftermath of the German occupation of Italy in the latter part of World War II. During this period, the country was under the Allied Military Government of Occupied Territories in southern Italy, as opposed to the German-occupied northern and central Italy, where the Italian Social Republic was established.

Strictly speaking, the term is used with reference to the period between September 1943, when King Victor Emmanuel III and the government fled Rome to Brindisi in the aftermath of the Armistice of Cassibile until Rome was liberated by the Allies on June 1944 and resumed its function as capital of Italy. However, its use is often extended to cover the period up to 1945 and the end of the war, that is, the entire period that Italy remained divided, during which time the Italian government, although it had re-established itself in Rome, still did not have full control of its nominal territory or local, police and military bodies. Administrative, military and political activities, and their documentation, were split between those managed by the government of Rome, by the Italian Social Republic, by the partisan forces and by the armies in the field.