The Modern Age adds a dimension your battlefield has never had to defend: the air. Fighters and bombers can reach any point on the map, which means your defensive thinking has to cover everything, everywhere — or accept risk deliberately.
Armor changes the ground game just as thoroughly. Tank formations move faster than anything medieval and hit harder than anything industrial, but they’re expensive, and anti-armor answers punish tank players who stop supporting their vehicles with combined arms.
The core Modern Age skill is layered defense: air-defense coverage over the places that matter, armor where breakthroughs threaten, and the judgment to know which parts of your empire you can afford to leave open. Missiles countering aircraft, and heavy tanks overpowering lighter vehicles, are the era’s defining matchup logic.