The US is a successful country and has almost always been at war.

The areas of the US that are most successful are those most insulated from social conflict. Areas that are subjected to state violence through overpolicing or are left to flounder in the face of industrial abuse, mafia violence, or unchecked domestic violence do much worse. Comparing Ferguson, MO to neighboring St. Louis illustrates this dynamic. One neighborhood is alternately brutalized by the city police and left exposed to domestic crime, dragging its socio-economic state into the gutter. The other is judiciously policed and socially supported by state and private largess, resulting in a far healthier and happier population.

Britain at its peak was holding 10s of countries at gunpoint.

And those countries suffered immensely. Meanwhile, Britain itself endured pockets of chronic crime and substance abuse specifically in areas that hosted military bases and other enclaves. The country saw an explosion in wealth inequality during its economic peak. Victorian England was a hellhole for the Dickensian proletariat.