Legend is that before his final battle defending Constantinople from the Ottoman empire, he took communion and then ran out to die in battle.
Was he right in taking communion, with the full intent of then going out to kill? Maybe you can justify that by saying he was defending his people, but what about if a Catholic king were to take communion before assaulting an enemy’s city or something?
That’s a separate question. If the war was unjust, then it would be right in my opinion for the Church to refuse communion. In fact, one of the things I admire about the Catholic Church is that it’s almost the only Christian church I can imagine actually doing something like that . . . .
But a purely defensive war against a ruthless enemy both of your nation and of your religion is pretty clearly justified, in my opinion, and I’m confident that this is also the teaching of the Catholic Church.
It is actually very common for soldiers to receive communion before battle, and Catholic chaplains give communion before battle all the time.