America runs a genuinely flexible resource economy — a unique Settler-replacement that claims and improves distant resources on demand, feeding into a set of Traditions that reward whichever yield you need most.
America's Modern-Age kit centers on resource management and adaptable Traditions like Gold Rush (Gold per assigned Resource) and Robber Baron (Influence per assigned Resource), plus a combat bonus for units facing more than one adjacent enemy and increased Military Production when your War Support outweighs your opponent's.
The Prospector is a unique Civilian unit that, when activated on an unowned Land Resource tile within range of a Settlement, claims a path of tiles back to that Settlement and improves the Resource immediately — a fast, flexible way to grab valuable land without waiting on normal border growth. The Marine is a unique Infantry unit with reduced Production cost and the Amphibious keyword, letting it attack straight off Embarkation with no Combat penalty and no Movement cost to embark or disembark.
The Industrial Park is a unique Quarter granting increased Food for every Resource assigned to the city. The associated Wonder, the Statue of Liberty, adds Happiness and spawns Migrant Units that can trigger a Growth Event in your Settlements; it must be built on a Coast tile adjacent to land.