War from the root level up depends on two elements: instinct of aggression(1) and presumption of superiority or preferably the invincibility(2).
We're all human and we understand instincts of aggression quite well, although maybe not always clearly defining the many sources of. Setting psychotic tendencies aside (illness, damaged dna), for example maybe you, the rare reader of this post, define yourself as a peaceful and friendly person, and as one may not realize how or why the massive immigration from Africa (and elsewhere, previously mentioned conflict zones) severely aggravates the nations that the immigration wave passes or targets. Basically it comes down to mainly xenophobia but also instincts of survival, both so basic that unless you have at least little bit of intelligence you likely succumb to the mass hysteria without being able to override it with reason. That is not to say that there won't be any threats to immigration, of course. But some of the immigrants are indeed running from the war and that brings up the second element of war-faring basics: presumption of invincibility.
Majority of conflicts in human history have happened between deeply or partially religious people, and in some cases if the soldiers were not religious to begin with, they probably sought help from the heavens under the rocket artillery fire anyway.
So what does this give us? The presumption of being guarded or shielded by the deities actively(1); the presumption or hope for divine intervention(2); resurrection/reincarnation - the ultimate invincibility(3).
The first is more of a morale boost and feeling of advantage, the second however is already more dubious as it can manifest as an enduring hope or take effect as an interpretation of some kind of signs seen on the battlefield, thus changing the course of battle.
And with most all religions providing some kind of safe passage to or from afterlife, abundant with pleasures or bonuses, it means the soldier can abandon most survival instincts depending on strength of belief and situation. Although, scientifically, the likelihood of being reincarnated as a blobfish is similar to being reincarnated as anything else. I'd say even that the likelyhood of you being reincarnated as blobfish is actually greater than your odds of coming back to life as something else, like a strong crap beetle, because reincarnation is load of bull designed to make you an obedient tool or at the very least shut you up about existential fears and insecurities.
While it is possible to design a philosophy or a culture that can substitute the benefits of religious beliefs, it is still very rare and difficult to achieve. At any rate, you'd still need that presumption of superiority or invincibility, likely granted by a cocktail of superior technology and quality propaganda.
If we however turn our focus back to the reasons for aggression, they do range from stealing your apple, coveting your neighbours farm field all the way to having wrong skin colour or too many riches. There is one other thing, though. Many religions support or can be interpreted to support aggression towards religiously different or indifferent individuals, both inside and outside the territory. From that we see added reasons for aggression, from the very significant second basis of warfare - one supports the other.
So in order to successfully wage war, you need to boost the already existing instincts of aggression with propaganda, cultivate it further through popularizing violence and tools of violence, and provide the community with a sense of invincibility and divine protection, preferably enhanced by indulging in the acts of violence.
But, there is also an important third aspect to warfare that assures the effectiveness of the former two already mentioned: low average intellect among the masses. An intelligent individual is better-equipped to counter fairytales and weigh risks vs benefits. While arguably a more efficient soldier, they are far less likely to become one. Aggression is the first fundamental concept to take a direct hit from increasing intellect while spirituality and religious beliefs endure far longer but not indefinitely. While conscription, propaganda and morale depends also on a sense of duty, a smart individual is able to begin to distinguish between the borders of absurdity and necessity. I don't want to add the urgency of defence into the discussion as it already assumes that one party is aggressively driven by the listed principles.
One could argue that significantly raising the educational and intellectual average among the population is too costly or physically impossible, but healthy amount of cynicism would point at the many benefits of having empty husks, including warfare. Truth is probably some sort of combination of those reasons.
