The international makeup of AMA SX and MX series in the last decade has been completely transformed by the number of foreign-born professional motocross racers making their home in the United States. For decades—although there were exceptions—if you were an American-born racer, you raced in America, and if you were from any other part of the world, you raced the World Championship Grand Prix circuit. Today, though, when you look down the starting line at…