In the aftermath of the fall of Mussolini in the first days of August 1943, communication for a cessation of hostilities began between the Italian government and the Allies. Negotiations became intense during the last days of August, finally leading to the agreement to sign a surrender in Sicily, a territory already occupied by Anglo-American forces.

The armistice of Cassibile was therefore stipulated on 3 September 1943 in Cassibile, a locality near Syracuse. It was signed on behalf of Italy by General Giuseppe Castellano, and for the Allies by US General Walter Bedell Smith, main associate of General Dwight Eisenhower. With this act, made public on 8 September, the Allies imposed an unconditional surrender on Italy: the Kingdom of Italy was to cease hostilities towards the Allied forces from that moment on. The agreement also stipulated that Italy would free all prisoners of war and allow the Allies to use its territory for the continuation of war operations against Germany.

The Italian requests, aimed at being able to negotiate the treaty in some way, were confronted with the Allies' determination not to concede anything: the 12 articles of the armistice in fact left very little margin of autonomy to the Italian government. The army was not informed of the negotiations in any way. Therefore, when the armistice was announced on 8 September, first by Eisenhower on Radio Algiers and later by a communiqué from General Pietro Badoglio broadcast by the EIAR (Ente Italiano per le Audizioni Radiofoniche), the Italian troops were completely without orders. They disbanded in what would characterise their subsequent fate: some soldiers joined partisan formations (in Italy as well as abroad), some surrendered to the German forces, while others refused to give up their weapons and preferred to fight.

Following the announcement of the armistice, Operation Achse was in fact triggered: in anticipation of the fall of Fascism and the possible Italian surrender, the German forces had planned to capture Italian soldiers and take possession of strategic locations in Italy to continue the war against the Allies.