Operation Diadem, also referred to as the Fourth Battle of Monte Cassino or, in Canada, the Battle of the Liri Valley, was an offensive operation undertaken by the Allies of World War II in May 1944, as part of the Italian Campaign of World War II. Diadem was supported by air attacks called Operation Strangle. The opposing force was the German 10th Army.

Operation Strangle was a series of air interdiction operations during the Italian Campaign of World War II by the Allied air forces to interdict German supply routes in Italy north of Rome from 19 March 1944 to 11 May 1944. Its aim was to prevent essential supplies from reaching German forces in Central Italy and compel a German withdrawal. The strategic goal of the air assault was to eliminate or greatly reduce the need for a ground assault on the region. The Allies failed in the overly ambitious objective of the campaign, namely the forced withdrawal of German forces from the Gustav Line, and did not curtail the flow of supplies but the air interdiction seriously complicated the German conduct of defensive operations and played a major role in the success of the subsequent Allied ground assault Operation Diadem.