In December 1940 Sperrle's air groups flew 2,750 bombing operations against British cities, more than the 700 flown by Kesselring's command. Manchester—in close proximity to Liverpool—was first bombed in December 1940, beginning the Manchester Blitz. From 22 to 24 December Manchester was bombed on two successive nights by 270, then 171 bombers. Northern targets were expanded to include participation in the Sheffield Blitz. On 12 December the bombing of the city opened with attacks by 336 aircraft then a further 94 two days later. The bombing was carried out over the course of nine hours, destroying much of the city. London remained a primary objective. On the night of 9 December 1940, for example, 413 bombers hit the capital and on 29 December the bombing caused what became known as the Second Great Fire of London. On this night alone Sperrle's air fleet dropped 27, 499 incendaries. It was not the heaviest raid on London—136 aircraft—but it was the most destructive.