First, Joan was a commander, but didn’t actually carry a weapon or fight in battle.
Second, there have been a lot of warrior saints or saints in the pipeline who were trained for battle if necessary, but actually did some kind of non-combatant job such as chaplain (Blessed Daniel Brottier) or firefighter (St. Florian).
Third, serving as a knight on behalf of your kingdom was kind of an expected career path for noblemen during a long period of time in Europe; it was how you protected your country and family, kept social order, and even got ahead economically. So we have a lot of saints who followed a normal path of military service for a while, like St. Thomas a Becket, St. Ignatius Loyola, and even St. Francis. None of them were canonized for stuff they did on the battlefield though, rather for stuff they did when they weren’t on the battlefield.
I think nowadays there are paths available for countries to resolve differences that didn’t exist in past centuries. But having said that, if some person was in a situation where they had to serve in the military (such as conscription, or to drive out invaders not friendly to the Church), it probably wouldn’t keep them from sainthood, especially if they ended up martyred.
Jose Sanchez Del Rio, who was just canonized recently, could be called a modern “warrior saint”; given that he was a 14-year-old flagbearer, he probably wasn’t running around killing people himself, but he was no doubt helping the soldiers in their mission to do so, by doing support work. He was fighting against anti-clerical forces and he was martyred pretty heroically for the faith.