Supply

Successful invasions aren't simply tactical wits and staying out of the way of enemy fire. There's Attrition to consider, and also the matter of Supply. Your Artillery and Siege weapons depend on Supply vehicles, and Supply depends on industry.

Smelters are the brains of your Metal gathering operation, and the home of Supply Technology. (You'll need territorial control over Mountains with well-placed Mines to build the muscle of your Metal ops.)

Research Supply three times throughout the game to increase the speed of vehicle production; increase the speed, capacity, and radius of your Supply vehicles, and decrease Attrition to your units while in enemy territory. The Quartermaster rarely gets the medals, but has the line on all the perqs.

Smelters from around the world in the mid game: