Wouldn’t then tanks be the ultimate choice vs. infantry, because of their range?
Tanks are important, but as the others have said you need plenty of infantry to absorb any losses.
What an IPM approach allows you to do is take territories without a battle. You leverage your shear numbers (both attack threat potential and defensive numbers) and force your opponent back. You only need enough offense to prevent your opponent from doing the same to you and in the case of Germany after round 1 you’ll have 4-5 ftrs, 1 bom, and a half dozen tanks or so. You don’t need any more offense. You need fodder!
So with IPM you want to start getting as many units on the board as possible and start shifting the deadzones out (moving them closer to Russia).
First you stack EE, then you look to move that stack to Bel. From here Russia has a choice, defend Kar or flee to Wrus. They will likely eventually move back to Wrus since that is far more important and the Bel stack can threaten Wrus directly as well. So now you’ve gained Kar and have successfully deadzoned it since any units that land there can be hit by your EE or Bel stacks. Wrus is usually a tough nut to crack so your next target is either the shift of your EE stack to Kar or the movement of the Bel stack to Ukr, EE stack to Bel, Ger stack to EE. Here you threaten Cauc and Wrus. Again Russia must choose which to defend and thus you’ll gain another deadzone and find you next target (either Cauc or Wrus).
B/c of your tanks movement you can station them in France (for defense) with the least amount of infantry possible to deter attack. Then as you move your infantry from Ger to EE to Bel to Ukr in 3 turns you can get your armor there in two. 1 turn to EE, 1 turn to ukr (or wrus). Although you may need to keep them in EE for protection and to help deter an assault on Fra. As you move your armor out of Fra, you may need to shift more ftrs there for defense. But their movement still allows trading on the Eastern front. Typically 2 ftrs in fra, 2 in EE would allow swapping of postions during each attack phase depending on where you need your strong defenders.
On the flip side Russia will be trying to push its way to Bel which will hold Germany progress at EE and then you just wait for the UK/US numbers to add up and push to EE. The Russian stacks will then go from Cauc/Mos to Wrus to Bel. So your armor should be stationed in Wrus. Not only to threaten Germany from advancing but can also reach the backside of Russia to protect against Japan.
The bottom line is it is cheaper to defend, 3 ipcs get you a defensive roll of 2. In order to “kill” that unit the other side must spend more than 3 ipcs. Larger scale battles put the ratio at 3:4 or 5 in terms of what the attacker needs to spend. So it is just cheaper and easier to move defensive stacks forcing your opponent to spend more to try and kill you. The simple way to look at it is to kill an inf you’d need at least 1 rt to attack. Even 2 inf plus planes costs 6 ipc. 😉