When the United States had conscription (during the civil war, two world wars and the first half of the cold war), anyone who was especially frum would have had an exemption as a “divinity student.” At all points in America, someone who was strictly frum (and unwilling to rely on heterim given for Jews serving in armies in which “being too frum” resulted in a firing squad) would end up being kicked out of the military as being “unsuitable” (not dishonorable, but one didn’t get to serve). Those who wanted to serve in the military (and remember that during the most of the 20th century, America’s enemies tended to be vicious anti-semites) would rely on various heterim that most Bnei Torah would not accept.
Two factors are important to remember. America has a long tradition of “citizen soldiers” and in fact professional full time soldiers were a minimal part of the military prior to the cold war. And going back to the 18th century, Americans have regarded religious tolerance as a core value encouraging the military to either discharge or accommodate those seen as “too religious” (dating back to Washington’s problems getting New England’s Calvinists and the mid-Atlantic and Southern states “Anglicans” to work together, leading to religious tolerance becoming an American way of doing things). If someone was drafted and refused to eat anything cooked or to do any military work on Shabbos, America would discharge them rather than hang them.