What role does religion actually play in the capacity for warfare? To suggest that faith is the primary driver of martial courage is to ignore the most striking examples of modern history.
Look at China. They are a secular, largely atheist society, yet their military and economic might is so formidable that few would dare to challenge them. Or consider the Japanese during World War II; while their culture was deeply traditional, their terrifying Kamikaze attacks were driven by a nationalist code of honor (Bushido), not a religious promise of an afterlife.
Furthermore, look at the historical resilience of Vietnam or the militaristic discipline of North Korea. Both are officially atheist states, yet are they any less capable as warriors? Clearly, the 'warrior spirit' exists entirely independent of a god.
The real divide isn't between the 'believer' and the 'infidel'; it is between the civilized and the hardened. History shows that people living in developed 'cities', surrounded by comfort and security, they naturally lose their edge as warriors. In contrast, those living in undeveloped, harsh, and unforgiving environments are forged into fighters by their surroundings, regardless of what they believe.
Those who insist on a religious explanation for military success need to undergo a 'mental recalibration.' They must abandon these comfortable delusions and confront the cold, hard facts of human geography and sociology.