I recently had a discussion with my roommate about the justification of collateral damage and he claimed that the bombardment of german cities like Dresden were irrelevant to the outcome of the second world war. I claimed that the german population served as the backbone of the Wehrmacht and that it was therefore justified to bomb big cities. Not necessarily to target the population but to destroy their infrastructure, break their will and disrupt their support for the frontline.
Well, it really depends on what the goal was.
As far as the goal of breaking the German population's willingness to fight and continue the war, strategic bombing accomplished next to nothing. Both The Crucible of War (the official history of the RCAF in WW2) and Weinberg's A World at Arms make the point that rather than breaking the spirit of the German population, it either created apathy or brought them closer together (although at one point Speer thought that the German resolve might be broken by it...post-war analysis is pretty clear that Speer was wrong).
As far as the goal of pushing forward the Allied grand strategy, it did quite a lot. By placing German cities under attack, the Luftwaffe was forced to keep a large portion of its strength in Germany for home defence, which prevented them from being used either on the Eastern Front or to oppose the Normandy landings. A huge portion of German guns also had to be redirected to air defence, as well as a large number of personnel to man those guns - all of which were prevented from being used on other fronts.
As far as destroying industry and infrastructure, there wasn't much success until 1944, although not through lack of trying - it's just really hard to hit something from 30,000 feet, no matter how good your bomb sights are. But, by 1944 Allied bombers had gotten good enough at it that Arthur Harris was surprised that it was possible for his planes to be that accurate.
That said, the V-rocket program was delayed a year because of the destruction of their industrial facilities through Allied bombing, so there were some notable successes prior to 1944. There just weren't many.
Okay. There are, to my mind, three separate questions here.
A third question then arises: "Was it morally justifyable to target production facilities in major urban cities knowing full well that high level strategic bombing would inevitably lead to major collateral damage?"
Let me start off by saying that there is no real consensus here. I think most historians will agree that the strategic bombing had an effect, however, how much of an effect and what kind of effect is very much open for debate.
Was the strategic bombing of Germany effective?
So, what we know with relative certainty is that the strategic bombing of Germany did not reduce Germanys industrial output of war materials. In fact, production of war materials actually rose significantly during the most intense periods of allied bombing (1944-1945). This is mainly due to the reorganization of the German war economy. Elimination of competing models, increased efficiency in existing production models and so forth.
It is reasonable to assume that had there been no bombing, production would have risen even further and key Nazi officials have admitted after the war that strategic bombing slowed certain projects (like the Type XXI submarine) and caused logistical difficulties in production and transportation. As to how significant this effect was, there is no clear consensus. While production did rise significantly it did not manage to meet most production goals set, in general falling about 30% short.
However, we cannot simply attribute the entirity of the shortfall to strategic bombing. The German war industry faced many issues such as loss of qualified manpower to the war, rapidly training new workers to replace those drafted, in some cases unwilling forced labour etc etc.
Did this significantly affect the outcome of the war?
To this there is a more clear answer although again, no complete consensus. Generally speaking, the answer is no. By the time the strategic bombing of Germany started having a noticeable effect on production output the war was very much lost. From 1944 and onwards the Axis did not have a snowballs chance in hell of actually winning the war. However, this is something we know with the benefit of hindsight. In 1944 this was far from apparent. In very simplistic terms, by the time strategic bombing was resumed in earnest Germany was already losing, it was only a matter of how long it would take. There is also a need to differentiate between different campaigns.
The Oil campaign was relatively successful and severely hampered the German war effort but exactly by how much is very hard to say for certain. The Luftwaffe never stood a chance aginst the combined might of the allied air forces in the first place so exactly how much of their defeat should be attributed to lack of fuel, lubricants and other petrochemical and synthetic products?
The same is true for the targeting of the chemical industry in general. It absolutely had an effect but how impactful that effect was to the downfall of Nazi Germany is debatable.
This brings us to the thrid an final part of the question. "Was it morally justifyable to target production facilities in major urban centers knowing full well that high level strategic bombing would inevitably lead to massive collateral damage?"
Again, there is no concensus here and perhaps it is ultimately not a question for historians at all. With the benefit of hindsight there is no doubt that the Axis powers would have been defeated even if not a single strategic bombing sortie had been flown throughout the entire war. However, stating that it is therefore morally unjustifyable is a historian's fallacy. It was far from apparent at the time and broadly speaking neither side was prosecuted for startegic bombing in the aftermath of the war. To quote Chris Jochnick and Roger Normand in The Legitimation of Violence: "By leaving out morale bombing and other attacks on civilians unchallenged, the Tribunal conferred legal legitimacy on such practices."
TL;DR - It had an effect, how significant is uncertain. However, it generally began too late to have been instrumental to the defeat of Nazi Germany and may at best have hastened the fall significantly thereby saving lives.
I covered a similar question here. I compared and contrasted the Blitz to the Allied bombing campaign of Germany and think you'll find a good deal of information to answer your question here.