The Battle of Anzio ensued after the Allied landings south of Rome, in the Italian port of Anzio area.
The Allies were trying to overcome the Gustav Line, and the port of Anzio was located just between the German defensive line and Rome. By landing at Anzio, the Allies intended to cut the line of communications of the Wehrmacht units which were defending the Gustav Line.
The Allied forces which landed at Anzio were made up of U.S. VI Corps, under American Major General John P. Lucas, and included, in the first wave, one U.S. infantry division, one British infantry division, three U.S. Ranger battalions and one U.S. parachute infantry battalion.
By Kirrages (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/deed.en)], via Wikimedia Commons
The Allied landings, codenamed Operation Shingle, started on January 22, 1944, and took the Germans by surprise, the Allies occupying Anzio and advancing a few miles inland. Expecting a German counterattack, General Lucas ordered his troops to consolidate their positions, which later brought him criticism because he did not use the opportunity to advance quickly. Field Marshal Albert Kesselring, commander of the German forces in Italy, had time to bring from reserve one panzer division, one panzergrenadier division, and one fallschirmjäger division.
Bundesarchiv, Bild 183-2000-0210-502 / Thönessen/Thönnessen / [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en], via Wikimedia Commons
The Allies did not assume the risk to occupy rapidly the mountainous area that separated them from their goal, Kesserling forces using later those excellent defensive positions.
By the end of the first week, both sides had received reinforcements. The first who took the initiative were the Allies, who attacked the Alban Hills, located in front of them, but failed to overcome the German defenses.
In February, the initiative passed to the Germans who managed to push the Allied troops close to the landing zones.
The naval and air support, as in the landing at Salerno, saved the Allies. Both sides had suffered heavy losses as a result of the fighting during the first weeks.
Afterward, the front of Anzio went into a deadlock up to May, when the Allies finally managed to break through the Gustav Line during the Battle of Monte Cassino. At the same time, on 23 May, the Allied forces at Anzio went into offensive.
The Allies initially advanced towards Route 6, endangering the German
forces retreating from the Gustav Line. However, on May 26, General Mark
Clark - commander of U.S. Fifth Army, which included VI Corps -
ordered the change of main attack direction to Rome.
His decision has remained disputed because the Allies missed the opportunity to cut off the withdrawal of the Wehrmacht units. On June 4, 1944, the U.S. Army entered the Italian capital, thus the Battle of Anzio ending with an Allied victory, even if it took much longer than initially estimated. The German forces, having the chance to retreat, later would oppose a strong resistance in northern Italy, on a new line called the Gothic Line.