Disclaimer: I'm a veteran
Our modern war machine is a complex monster of politics, money, nationalism, and power.
This is a focus on nationalism.
How do you get and keep a society to love war? How do you staff your army with young males eager to kill?
Simple:
You force your young males to join a draft when they turn 18. This gives the politicians a list of candidates to send to war.
The second is a little harder and takes enormous Public Relations work, but we have succeeded in the West at it. The second thing you must do is celebrate the soldiers as if they are celebrities. You march them in parades, thank them on the fourth of July, and bang the drum of 'We support our soldiers'.
This is crucial to the narrative of modern war.
Step 1: Force your young males to join
Step 2: Celebrate their existence.
Wash, Rinse, Repeat