Hitler took direct control of Russia offensive overruling the operational generals (whose superiors feared Hitler and were making mistakes in their craven support of him). Long supply lines, the brutal winter :- both were impediments … ; the unnecessary declaration of war on US: perhaps tactical focus on Moscow alone rather than pushing also elsewhere at same time could’ve pushed them to win Moscow; and no declaration on US would’ve helped … but in the end these are exactly tactical arguments. As is Eloquently stated above: strategically they could not win: was it inevitable? By lack of resources yes, but the ruthless soviet commitment (and bravery) was stalling the German army effectiveness by a sheer numbers game (a sobering lesson to recall) … : In the end economic might and a war on 2 fronts … Germany were declining and the end game was set in 1941; and leadership was crucial: Churchill / Roosevelt turned US opinion…to support brutal Stalin: all whilst the increasingly isolated Hitler depended on personal control : his erratic decisions were both the cause of the war and the reason Germany lost. If his ambitions were less he could have held more for longer … but that simply wasn’t an option for him/his ideology.