Attrition

Few defensive stratagems are so elegant as Attrition damage technology. If you're playing as the Russians, you have Attrition built right in to your nation, in the form of Russian Winter - as soon as enemies step into Russian territory, they start to wither and die, even before you fire a shot.

All 18 nations can strengthen the power of Attrition within their borders, however; even the withering Russian motherland grows fiercer and bleaker with the expense of a few rubles (and some Metal) at the Tower.

Spend the money and even enemy forces already across your borders will start to suffer.

French Supply vehicles can heal their troops, but all other nations' armies must return home to heal. Reinforcements are considerably faster, but of course they're considerably more expensive.

If you must choose between adding a couple units to fight an invasion or researching Attrition, the choice is clear. Attrition turns battles.