Why were the main cities of Britain bombed by Germany in the 1940-1941?

The bombing of Britain was carried out for two reasons.  The first was to destroy military capabilities, communications centers, rail lines and industrial capacity, and thus strike against the ability of Great Britain to maintain governmental and military cohesion and the capacity to wage modern industry-based warfare.  The second reason was to terrorize the population. Although much rhetoric has been spouted throughout history about war being waged on soldiers and not civilians, the truth is that from earliest human history the general population has always been considered a legitimate target of war.

London, of course, was the main target, but Liverpool as the main port city of the west coast was also a major target throughout the war.  Most port cities and industrial centers were heavily attacked, including Manchester, Newcastle, Edinburgh, Dundee, Glasgow and the Northern Irish shipbuilding center of Belfast.  The rest of Ireland was not bombed by the Germans, as the Republic of Eire was a neutral country.  Leeds, Sheffield, Coventry and Birmingham were also heavily attacked, some 554 persons killed in a single 1940 raid on Coventry.  In September of 1940 over 13,000 were killed in London alone.  Nearly 30,000 Londoners were killed during the war in German bombing raids.  Nearly every city of any consequence at all in Britain was bombed.

Ironically enough, it was Britain which initiated what was termed "city-bombing", striking targets inside Germany on the night May 11, 1940.  Although British policy stated officially that such bombing was "absolutely contrary to international law," Britain bombed Berlin with a force of 81 bombers August 25, 1940, and after five further raids the Germans began their "blitz" some three weeks later.  However, the Germans had initiated bombing of civilian centers during the Spanish Civil War in 1936 at Guernica, the subject of a famous painting by Picasso.  The practice of aerial bombardment of civilians was first begun by the Red Army during the Russian Civil War immediately following WW I.

The heaviest losses of life from aerial bombardment during the Second World War were among civilians, in the fire bombing by the Allies of Dresden, Germany, and the fire bombing of Tokyo, Japan by American forces.  The fires in Tokyo killed some 130,000 persons in one raid, about the same casualty number as the Dresden bombing, and more than those killed in the atomic bombings.