A terrible, horrible tragedy. Stalin had the Red Army sit just outside Warsaw until the Germans had defeated the Home Army. He was determined that the Soviet puppet Lublin Polish government would rule post-War Poland, not the Government-in Exile in London.
It also has to be said that at this stage of the war the Nazis had nothing to gain by crushing the uprising (nor expend resources in the process) and in fact politically much to gain by pulling back and letting the Poles declare a free government.