Operation Achse (German: Fall Achse, lit. 'Case Axis'), originally called Operation Alaric (Unternehmen Alarich), was the codename for the German operation to forcibly disarm the Italian armed forces after Italy's armistice with the Allies on 3 September 1943.

Several German divisions had entered Italy after the fall of Benito Mussolini in July 1943, while Italy was officially still an ally of Germany, despite the protests of the new Italian government under Pietro Badoglio. The armistice was made public on 8 September. German forces moved rapidly to take over the Italian zones of occupation in the Balkans and southern France, and to disarm Italian forces in Italy.

Some Italian troops, with no orders from superiors, and hampered by desertions, resisted the Germans. On the Greek island of Cephalonia, 1,315 Italian soldiers were killed in action against the Germans and over 5,100 Italian soldiers from the 33rd Infantry Division "Acqui" were summarily executed by the German Army after running out of ammunition and surrendering. In Rome, with the royal family and the government having fled, a disorganized defense by Italian troops of the capital was unable to withstand a German attack. Some individual soldiers and sometimes whole units, like the 24th Infantry Division "Pinerolo" in Thessaly, went over to the local resistance. Only in Sardinia, Corsica, Calabria and the southern part of Apulia were Italian troops able to offer successful resistance and hold off the Germans until relieved by the arrival of the Allies.

The first German combat units were sent to Italy to bolster its defenses against a probable Allied attack on Italian soil. Germany and Italy were still allies. The decision to create German units in Italy was made during the final phase of the Tunisian campaign; on 9 May 1943, two days after the fall of Tunis to the Allies, the German High Command (OKW) informed the Italian Supreme Command (Comando Supremo) that three new German units would be formed, mostly employing second-line German units evacuated from North Africa. They would be the Sardinia Command (later 90th Light Infantry Division), the Sicily Command (later 15th Infantry Division), and a "ready reserve". Adolf Hitler wrote to a dubious Benito Mussolini that since they were weak units that needed reinforcements, two additional German divisions would be sent from France. The 1st Fallschirm-Panzer Division Hermann Göring arrived mid-May 1943 and was sent to Sicily, and the 16th Panzer Division arrived in early June and was sent west of Bari. On 19 May also, the headquarters of general Hans Hube's XIV Panzer Corps was also sent from France to strengthen the command structure of the Commander-in-Chief South (Oberbefehlshaber Süd), Field Marshal Albert Kesselring.

On 20 May 1943, during a prolonged discussion at his headquarters, Hitler expressed his doubts about the political stability of the Fascist government and the danger of a collapse of his Italian ally. A report by the German diplomat Konstantin von Neurath found declining morale among the Italian population and pro-British sentiment spreading through the professional classes and the military. Hitler was convinced that the situation in the Mediterranean needed great attention, and a detailed plan had to be prepared for a collapse of Italy or an overthrow of Mussolini. More reports about a speech delivered by the Italian diplomat Giuseppe Bastianini, information from Heinrich Himmler's men in Italy and the presence in Sicily of General Mario Roatta, who was considered untrustworthy, strengthened Hitler's suspicions.

On 21 May, Field Marshal Wilhelm Keitel, the head of the OKW, issued guidelines developed to respond to the possible defection of Italy from the Axis. The plan envisioned a series of operations in different theaters: Operation Alarich, the invasion of the Italian mainland; Operation Konstantin, the neutralization of the Italian forces in the Balkans; Operation Siegfried, occupation of the Italian-occupied areas in Southern France; Operation Nürnberg, to guard the France–Spain border; Operation Kopenhagen, to control the passes on the France–Italy border.

Meanwhile, German reserves kept being redeployed to face potential threats in the Mediterranean theater. Hitler, seriously worried about the Balkans and in conflict with the Italian leadership and Mussolini himself because of collaboration agreements between the Italian and local partisan forces, decided to send the 1st Panzer Division to the Peloponnese and even considered sending to Italy his three elite Waffen-SS armored divisions, then deployed on the Eastern Front for Operation Citadel.

On 17 June Mussolini, after a partial refusal, urgently asked for two German armoured divisions, as a reinforcement to confront the powerful Allied forces. After more arguments caused by another change of mind by Mussolini and by a proposal by General Vittorio Ambrosio, the Chief of Staff of the Italian armed forces, to turn down German reinforcements and to move to Italy the Italian troops deployed in France and the Balkans, the ever-deteriorating situation (during Operation Corkscrew, Pantelleria surrendered without resistance on 11 June) induced Hitler to send three more German divisions: the 3rd Panzergrenadier Division, the 29th Panzergrenadier Division (both newly reconstituted in France after their decimation at Stalingrad), and the 26th Panzer Division. The last of them was deployed at Salerno on 9 July. The 29th Panzergrenadier Division was sent to Foggia in mid-June and the 3rd Panzergrenadier Division was deployed north of Rome in the first days of July. Meanwhile, on 24 June, the Reichsführer-SS Brigade had been moved to Corsica, and in mid-July, the command of the 76th Panzerkorps (General Traugott Herr) also arrived.