Through the western location of the city and the existence of the Rhine as a good navigational aid, Cologne was a favorite target for the Allies in the war. On 12 May 1940 the first small air attack took place on Cologne, on the night of March 2, 1941, the first large attack with about 1000 bombers occurred. On March 2, 1945, there was the last of a total of 262 air raids. This was once again particularly violent, because the Allies wanted to clear the way for the advancing groups of troops in the truest sense of the word.
During this entire period, there were 1,122 airliner alarms and 1,089 "public air warnings" (which had been introduced as a new alarm signal in August 1942). These alarms caused the population to spend up to 2,000 hours in air-raid shelters or basements, equivalent to 83 days and nights or nearly three months. If the local broadcasters suddenly stopped transmitting, this meant that an attack threatened, because attacking units were oriented on radiosignals, and therefore they were switched off in the event of the detection of advancing enemy units. People fled into the shelters and cellars after the sound of the sirens. Especially at the beginning of the air war there were not many public shelters. When a house was hit, survivors could also flee to neighboring cellars through connecting openings. Frequently, however, every possibility of escape had been spilled, cellars collapsed or smoldered by fire so that there was no escape for the people. They were slain by the rubble in collapsed cellars or stifled agonizingly. Of the war dead in Cologne two thirds died in the shelters and cellars.
The biggest destruction of Cologne during the Second World War came about by the first 1,000 bomber attack of the war - cover name "Operation Millennium". Exactly 1,096 bomber aircraft started from Allied airfields and flew a violent attack on the city in the night from the 30th to the 31st May 1942 between 0.47 and 2.25 clock. Until shortly before the start of the operation, Hamburg was also seen as a possible alternative destination, but bad weather over northern Germany left the decision in the short term to Cologne. 243 ha / 2.4 sq km of the city were put into rubble during this major attack, about 30,000 houses damaged or destroyed. Only 300 houses survived the attack undisturbed. About 1,500 tons of bombs fell on the city. Two thirds of the dropped bombs were fire bombs, which then caused about 12,000 individual fires in the city, leading to 1,700 major fires. Because of the fact that many inhabitants had already left the city during the war, there were "only" about 480 dead. In addition, there were 5,000 injured, there were 45,000 homeless people.
Even though many of the fires had already been caused by this attack, there was the phenomenon of the firestorm for the first time in a major attack on June 29, 1943, counting about 4,400 dead. After this attack, there were already 230,000 homeless people in the city.
At the beginning of the war about 770,000 citizens lived in Cologne, in the end only 40,000. The air raids resulted in a total of about 20,000 deaths. During the entire air war, about 1.5 million bombs fell to Cologne.