In August 1940, Luftwaffe night bombers aiming for RAF airfields, accidentally bombed London killing civilians. The British responded by bombing Berlin.
From September 1940 to May 1941 the Luftwaffe attacked 16 British cities. London was attacked 71 times in 37 weeks. More than one million London houses were destroyed or damaged, and more than 40,000 civilians were killed.
Great pix:https://www.theatlantic.com/photo/2011/07/world-war-ii-the-battle-of-britain/100102/
http://ww2today.com/14th-october-1940-disaster-at-balham-tube-station
A sad story of the Blitz
https://www.iwm.org.uk/collections/item/object/1060022009
https://www.dailymail.co.uk/news/article-1342305/The-Blitzs-iconic-image-On-70th-anniversary-The-Mail-tells-story-picture-St-Pauls.html