Stalin had been pressing for a new Western Front in Europe since 1942, and the British and Americans were still delaying while they prepared for it. The Soviets felt like they were being abandoned to fight the Germans on their own.
"30 December-1December 1943. The first from the ‘Large A few’ war time meetings. It had been below of which Churchill became uncomfortably aware about the level this agreement Uk might had been regressing in relation to the partners. "