You are probably thinking of Dresden, which is the most infamous case, but there were others.
The bombing of cities in Europe during WW II really needs to be put into context and is a very complex moral and military subject that continues to generate student papers at the War Colleges today (I wrote a few myself). Just a few observatons:
---Germany and Britain were engaged in a no-holds barred struggle for survival. There was no doubt that a German victory would have meant virtual enslavement of the Brittain. When the stakes are high, the gloves come off. As an historical note, the UK was well along in the development of anthrax bombs for use against Germans on the Continent if the need arose.
--- While Britain was bombing these cities, the Germans were doing their very best to do the same to the UK. Memory joggers: Coventry, V-1, V-2, blitz.
--- There was nothing especially barbaric about aerial bombardment in comparison to other means of warfare during WW II. The city of Stalingrad was as thoroughly destroyed by artillery and troops up close as any city targeted by bombers--it was the bloodiest battle in human history. Sending hundreds of thousands of US troops to Iran, besides being crazy, would not likely result in less Iranian carnage than other means of attacking targets there.
--- As a practical matter, the UK was on the ropes in the early years of the war. Germany was at the door and the UK did not have a toehold on the Continent from which to attack. Aerial bombardment (and the Royal Navy) were the only means to stave off Germany while buiding strength for an eventual invasion of continental Europey. The US favored "precision" daylight bombing to target particular German industrial targets deemed crucial to the war effort. Due to practical limitations in being able to strike these targets with precision and due to errors in determining exactly which nodes were truly critical to the German war effort, this US campaign had limited success. Still, the US deserves a lot of credit for attempting to minimize civilian casualties while making the most of the limited air power tools of the time. (And, believe it or not, there was considerable debate about the morality of targeting German cities at the time, even given the nature of the war). American aviators paid a heavy price for attempting to implement daylight precision bombing, and it did produce some noteable results that impacted the war effort, particularly with respect to German POL production and some portions of the transportaton system. The RAF decided early on that daylight bombing was going to cost them a lot of lives and would be limited in effectiveness. and began a campaign of night bombing of German cities (effectively, only area targets could be hit at night). The US also helped them in many of these raids, it was a combined offensive. While there was some hope that the raids might break German morale (which we might today classify as a"terrorist" objective), the primary officialy objective of the raids was also to displace German war workers and disrupt production. Now, Dresden is more difficult to explain in this light, as it was not a major industrial city. Still, it did have important rail facilities and there was some belief, as it had not been struck up to that time, that it was being viewed by the Germans as a sanctuary and that war production was likely being moved there. This is tenuous rationale at best. But, placed in the context of the time, the Dresden raid is at least understandable. Take a look at pictures of 1943 London, imagine you are driving through that on the way to an RAF targeting meeting, and tell yourself that you would stand up and tell everyone Dresden was too pretty to bomb. Not gonna hapen.
Note: The fire bombing of Tokyo (a "conventional, non-WMD" attack) killed more people than the atomic bombs dropped on Hiroshima or Nagasaki. It doesn't take a WMD to kil thousands of people, but it does make it easier and more difficult to defend against.
Does this have anything to do with Muslim terrorists? I don't think so, but maybe they do. In general, targeting of civilians does not produce the results expected by the atacker--it hardens attitudes, strengthens the will of those attacked, and makes t more difficult to achieve results. Still, if the war of civilizations (modernism, rationalism and the scientific method, respect for the dignity of the individual vs. a barbaric 6th century theocracitic world view) ever makes it necessary or useful to engage in widescale killing (and to accept civilian casualties of our own) to preserve our civilization, the West will do it. I hope.
