The countries engaged in WWII didn't just differ in the technology they put into their weapons of war, they also had very different training strategies and different combat doctrines that they followed. So if you just swapped a Panzer for a Pershing, you'd still have a different way that the tank was used in combat due to different tactics. In Tanks, from Gale Force Nine, you'll be able to distinguish your forces a bit more with Doctrine cards, of which we get a preview today.
These Doctrine cards are a new type of upgrade card for your tanks. It's not equipment or necessarily training, but just a more about how the different nationalities fought in war and how they would approach tactics, in general. A specific tank can only have one Doctrine card assigned to it at a time, so you'll have to decide what best fits your particular force when you build it.