Remember it was Stalin who said "the death of a single man is a tragedy, the death of a million is a statistic". While the death of "innocent civilians" in war is tragic, this has been going on for a very long time and is not just a feature of modern/nuclear warfare although starvation, being raped to death, death as a slave, death through epidemics tended to be the mode of death for civilians up until relatively recently. The deaths of the civilians of Dresden and Hiroshima were not on the Allied politicians/military leaders who ordered those strikes. They were on the leaders of Germany and Japan who started the war, and who had no compunction about using these sorts of tactics to begin with. If there is a shootout during a crime and, in the course of this, innocent bystanders are killed, their deaths are charged to the criminals who began the whole episode. No attack on Poland, no Dresden; no Pearl Harbor, no Hiroshima. On the "micro" scale international law is clear, if party "A" uses civilians as a shield during an assault, and in resisting said assault party "B" causes civilian casualties, it is "A" who has committed a war crime not "B".
In 1948 the USSR is still in terrible shape. The countries of Eastern/Central Europe are not much better - especially Poland and East Germany which are the corridors through which all Soviet troops and materiel must pass. The ability of the Soviet Navy to interdict the flow of goods and troops to Europe from the Western Hemisphere is close to zero, they have few submarines capable of getting to the shipping lanes, and the Allies can draw upon active and former naval personnel with vast experience in ASW. Certainly the USSR can shoot down B-29s and B-36s, however their air defense system is primitive and they have essential zero experience in dealing with strategic bombing. Sure, SAC needs work but they have the planes, and the bombs to learn. Drop a couple of 40KT bombs on Baku, and Soviet petroleum supplies drop precipitously. Hit some key rail junctions, and likewise for transport to the front, and Leningrad simply cannot avoid getting hit as well as cities in Western Ukraine - giving Moscow a pass for the moment.
In the US and UK there would be no support for UNTHINKABLE in 1948, but in response to the USSR starting the war sure.