That's a silly comparison:
a) During the First World War, Germany had no choice about a two-sided conflict (also, they knocked Russia out of the war, and before the intervention of the US, they had, in essence, won the conflict on the Western Front); and
b) During the Second World War, the Soviet Union was poised to invade the West (you can debate the merits of that argument), and AH was at a loss as to why the UK refused to negotiate a peace (since by any rational perspective, they had already lost the capability of reengaging the Germans on the continent).
P.S. Also, in addition to being totally irrelevant to the conversation, it's rather comical that the Germans are constantly bashed whereas the Russians and Americans are ignored for using the same tactic (and that's not even taking into consideration the true nature of the conflict with its multifaceted fronts).
P.P.S. As for the Iron Horde, people are overplaying the nature of the threat to the Iron Horde pre-invasion. You can chalk it up to bad writing, but there's no-way that a single clan - coupled with a decimated Draenei population - posed a threat. In essence, WoD would have been substantially better if they had extended the story arc into two expansions (that's coupled with Gul'dan betraying the Iron Horde and the player experiencing a global conflict on a second continent, e.g. Farahlon, Goria, or both).