Prevent Hitler from invading Russia. In other words, force him to finsh what he started in Europe. Then I'd see what would happen from there. I suspect Russia would eventually fall without support and the allies forcing the German army to be split in two, and America would be invaded shortly after. Now that would be intresting.
If that isn't possible, then keep Rome from falling intially. Again, it would eventually collapse, but how long would it have kept going without the fear of horde attacking it.
If that isn't possible, then keep Rome from falling intially. Again, it would eventually collapse, but how long would it have kept going without the fear of horde attacking it.