veterans of the country’s 21-century “forever wars” — men and women who lost buddies and limbs to roadside bombs and suffer psychic scars — struggle to understand the why behind them. Some wonder: Were they instruments of less-than-noble imperialist adventures?A century ago, a gimlet-eyed Marine who featured in pretty much every early U.S. empire-building expedition — in Cuba, the Philippines, Panama, Mexico, Nicaragua and Haiti — asked himself the same question.