War against the military is one thing war against civilians is another and "You hit me first" not a valid justification
From February 13 to February 15, 1945, during the final months of World War II, Allied forces bombed the historic city of Dresden. Dresden was neither important to German wartime production nor a major industrial center, and before the massive air raid of February 1945 it had not suffered a major Allied attack. By February 15, the city was a smoldering ruin and an unknown number of civilians—estimated at somewhere between 35,000 and 135,000–were dead.
An important aspect of the Allied air war against Germany involved what is known as "area" or "saturation" bombing. In area bombing, all enemy industry–not just war munitions–is targeted, and civilian portions of cities are obliterated along with troop areas. Before the advent of the atomic bomb, cities were most effectively destroyed through the use of incendiary bombs that caused unnaturally fierce fires in the enemy cities. Such attacks, Allied command reasoned, would ravage the German economy, break the morale of the German people and force an early surrender. The Allies never overtly admitted that they were engaged in saturation bombing; specific military targets were announced in relation to every attack. However, it was but a veneer. In the heat and anger of a World War few mourned the destruction of German cities that built the weapons and bred the soldiers that by 1945 had killed more than 10 million Allied soldiers and even more civilians.
Now, as Dave pointed out, Germany was indeed the first to employ area bombing tactics during its assault on Poland in September 1939. In 1940, during the Battle of Britain, the Luftwaffe failed to bring Britain to its knees by targeting London and other heavily populated areas with area bombing attacks. One cannot deny that the Royal Air Force was avenging the bombings of London and Coventry in 1942 when it launched the first of many saturation bombing attacks against Germany. In 1944, Hitler named the world's first long-range offensive missile V-1, after "vergeltung," the German word for "vengeance". The "V" weapons were an expression of his desire to repay Britain for its devastating bombardment of Germany.
Dresden was called "the Florence of the Elbe" and was regarded as one the world's most beautiful cities for its architecture and museums. Dresden's contribution to the war effort was minimal compared with other German cities. In February 1945, refugees fleeing the Russian advance in the east took refuge there. As Hitler had thrown much of his surviving forces into a defense of Berlin in the north, city defenses were minimal, and the Russians would have had little trouble capturing Dresden. It seemed an unlikely target for a major Allied air attack.
On the night of February 13, hundreds of RAF bombers descended on Dresden in two waves, dropping their bombs indiscriminately over the city. The city's air defenses were so weak that only six Lancaster bombers were shot down. By the morning, some 800 British bombers had dropped more than 1,400 tons of high-explosive bombs and more than 1,100 tons of incendiaries on Dresden, creating a great firestorm that destroyed most of the city and killed numerous civilians. Later that day, as survivors made their way out of the smoldering city, more than 300 U.S. bombers began bombing Dresden's railways, bridges and transportation facilities, killing thousands more. On February 15, another 200 U.S. bombers continued their assault on the city's infrastructure. All told, the bombers of the U.S. Eighth Air Force dropped more than 950 tons of high-explosive bombs and more than 290 tons of incendiaries on Dresden. Later, the Eighth Air Force would drop 2,800 more tons of bombs on Dresden in three other attacks before the war's end.
Because there were an unknown number of refugees in Dresden at the time of the Allied attack, it is impossible to know exactly how many civilians perished. After the war, investigators from various countries, and with varying political motives, calculated the number of civilians killed to be as little as 8,000 to more than 200,000. Estimates today range from 35,000 to 135,000. Looking at photographs of Dresden after the attack, in which the few buildings still standing are completely gutted, it seems improbable that only 135,000 of the million or so people in Dresden at the time were killed. Cellars and other shelters would have been meager protection against a firestorm that blew poisonous air heated to hundreds of degrees Fahrenheit across the city at hurricane-like speeds.

American author Kurt Vonnegut, was a prisoner of war in Dresden during the Allied attack and tackled the controversial event in his book "Slaughterhouse-Five"