Dresden was a German city that was a target of concentrated firebombing by American and British Bomber Command during WW2. American and British Bomber Command often disagreed on targets, both in purpose and in choice - American Bomber Command in Europe prioritized industrial targets; British Bomber Command preferred 'terror bombing' in retaliation for Nazi bombers over British cities (American Bomber Command over Japan embraced terror bombing). Dresden, however, was both a large industrial and logistics hub and a prominent German city; furthermore, the Soviets requested that Dresden be targeted to relieve pressure on the Eastern Front, as many Nazi troops and equipment passed through Dresden on their way to the Eastern Front.
Dresden was... obliterated. The full force of both the British and American air forces in Europe were concentrated on the city, and the firebombing was so intense that it created self-sustaining wind systems during the burning - a phenomenon known as a 'firestorm'. Naturally, this is a very distressing event, and many of the civilians in Dresden remembered the situation with horror.
Nazi propaganda played up the bombing, however, increasing the claimed civilian deaths by an order of magnitude (~25,000 dead in reality - 200,000 claimed by the Nazis), and the revulsion towards firebombing and attacks on cities in the postwar period has led many to embrace such revisionism to claim that not only was Dresden unnecessary (debatable, but a legitimate objection) but a defenseless and militarily irrelevant target (both demonstrably untrue).