Complex sentence: It was intended that the army would be supplied by air but the quantity of supplies necessary was far beyond the ability of the "Luftwaffe" to carry, the strength of Sixth Army diminished and the Soviets gained the upper hand inside the city.
Split into simpler sentences: It was intended that the army would be supplied by air, but the quantity of supplies necessary was far beyond the ability of the "Luftwaffe" to carry. The strength of the Sixth Army diminished and the Soviets gained the upper hand inside the city.