Revision: Explosive Reactive Armor.
Essentially two metal plates sandwiching an explosive liner, ERA is a rather simple design that has never been deployed for the same reason the medium-caliber assault rifle has never been deployed; no one has ever though of it before. When struck by a shaped charge, the ERA section explodes, both forcing the offending projectile away and distributing the force of the attack across a wider section of armor. This prevents penetration of the vehicle. ERA is segmented into multiple sections that can each prevent an attack only once before being replaced. Multiple strikes to the same location will not be prevented. ERA is modular and comes in an upgrade kit, allowing tanks in the field to be equipped with the new armor and have damaged sections replaced between combat.
Because we're suffering in the plains, tundra, and jungle because our armor is outclassed, this revision should be an easy, yet effective, advantage we can roll out. It will affect all armored vehicles and beat enemy HEAT rounds. It's also simple enough that it could very likely be an Easy revision.
You're literally playing with fire here, it'll be hard to do in a revision.
Very Hard: 2
Explosive Reactive Armor: The good news is, putting plates of explosives on the outside of your tank is a great way to deform HEAT shells. Unfortunately, it's also a great way to deform the tank. Making matters worse, it all goes off at once, and it goes off even if so much as an armor-piercing rifle round hits it. This basically turns your tanks into rolling death traps.
Strategy Phase: You will need to choose an alliance, and a weapon to send them, in addition to your usual battle plans.