Before the war, Britain had identified Germany's reliance on oil and oil products for its war machine, and the strategic bombing started with RAF attacks on Germany in 1940. After the US entered the war (December 1941), it carried out daytime "precision bombing" attacks – such as Operation Tidal Wave against refineries in Romania in 1943. The last major strategic raid of the European theater of the war targeted a refinery in Norway in April 1945. During the war the effort expended against POL targets varied, with relative priority sometimes given to other objectives such as to defeating the German V-weapon attacks or to preparations for the invasion of western Europe in 1944.