The Battle of the Ruhr was a 5-month long campaign of strategic bombing during the Second World War against the Nazi Germany Ruhr Area, which had coke plants, steelworks, and 10 synthetic oil plants. The campaign bombed twenty-six major Combined Bomber Offensive targets. The targets included the Krupp armament works (Essen), the Nordstern synthetic-oil plant (Gelsenkirchen), and the Rheinmetal-Borsig plant in Düsseldorf. The latter was safely evacuated during the Battle of the Ruhr. Although not strictly part of the Ruhr area, the battle of the Ruhr included other cities such as Cologne which were within the Rhine-Ruhr region and considered part of the same "industrial complex". Some targets were not sites of heavy industrial production but part of the production and movement of materiel.
Although the Ruhr had always been a target for the RAF from the start of the war, the organized defences and the large amount of industrial pollutants produced that gave a semi-permanent smog or industrial haze hampered accurate bombing. Before the Battle of the Ruhr ended, Operation Gomorrah began the "Battle of Hamburg". Even after this switch of focus to Hamburg, there would be further raids on the Ruhr area by the RAF in part to keep German defences dispersed, just as there had been raids on areas other than the Ruhr during the battle.