And many who actively served in WWI and WWII didn't either.
At the heart of trauma is helplessness. That is why it tends to cement in childhood when you are truly helpless. You don´t have your own home to run to, or your own money to buy food etc. And, you're small and weaker than so called "adults".
If a soldier didn't feel helpless while defending their country there's a good chance they haven't developed trauma.
Trauma then is a form of disregulation in the nervous system, a conditioning, a form of imprint where the nervous system routinely "triggers" in sympathetic activation, fight/flight/freeze.