It takes a great deal of training to turn the average person into a willing and able killer, and during every war on record there have been significant propagan

It takes a great deal of training to turn the average person into a willing and able killer, and during every war on record there have been significant propaganda efforts (or religious indoctrination) to make sure the enemy is seen as less than human. Furthermore, the soldiers who serve in the armies of the nations you mention who actually have to do any killing often end up with PTSD and sky-high suicide rates. Humans may be more naturally hesitant to kill their fellow human beings than you think.