Didn't someone just ask this a few days ago?

Firstly, as mentioned in the thread from a few days back, a war crime is a legal term, not a moral one. So in simple terms, the raid on Dresden was not a war crime, as it was never brought before court as such. To look at it from a moral/strategic point of view then the matter is a little less set in stone. By this point of the war, the Soviet Red Army was beginning its advance into Germany from the East. Dresden lay in the path of a major element of the Soviet advance. As a major regional centre for communications and transport, it represented a significant obstacle to the Red Army. As a show of commitment to the Soviet cause in this crucial time, the Western Allies took up the cause of nullifying this obstacle. The resulting raid was suitably destructive - although German reports place the casualties at wildly dispersed extremities, it is likely that around 20,000 civilians were killed over the course of the RAF's nighttime raid and the daylight follow up by the USAAF. It's worth noting that Dresden's high civilian casualties is not an isolated instance - cities such as Hamburg and Köln suffered similar fates. In summation, whilst the loss of civilian life in Dresden was significant, it was not the wanton massacre it has become popular to portray it as.

Further reading/sources: Men of Air - Kevin Wilson, Reaching for the Stars - Mark Connelly, The Berlin raids - Martin Middlebrook, The Second World War -W.S. Churchill