Digest of terms
it has become customary to distinguish holy war from just war, but the distinction is both relatively modern and blurred. It may be helpful to distinguish between justification and motivation.
Motives for going to war are always going to be mixed, and for religious people religious motives will operate both in what are called holy and in what are called just wars; yet in both cases religious people will (or should) be motivated by wanting to do what they regard as being morally right, so moral motives will be involved too. Thus holy war is not properly distinguished by being labeled exclusively religiously motivated warfare, much less by being able to identify religious motives in the mix - for many people that would have included World War 1, a war not remotely typical of 'holy' war.
What may perhaps be different is the justification in the two cases, just war being sanctioned by moral considerations separable from religion, while justifications for holy wars must go further and invoke specifically religious reasons as essential - and, usually, dominant. Prime examples are the Israelites' conquest of the 'holy land' as told in the Bible, and the Muslim conquests of the Middle East and North Africa, both of these being justified in terms of a specifically religious goal - of securing a 'holy' land for a divinely chosen people in the one case, of safeguarding and spreading God's word in the other.
The Crusades are also commonly taken to have been holy wars, and certainly there was strong religious motivation - papal promises of forgiveness of sins. Perhaps, though, the justification - retrieving Christian control of the land made doubly holy by Jerusalem as the site of Christ's crucifixion and resurrection - is the more telling.
The Jewish idea of holy war provided potent symbolism for the battle between good and evil (itself derived in part from Zoroastrian religion), or for the final battle envisaged by many as going to take place between Jews and Gentiles at the climax of history. The theme was taken up into Christianity. Common to both holy war symbolism and holy war reality in these cases is a readiness to envisage and sanction ruthless massacre of enemies. In Muslim jihad, by contrast, conquerors prioritized conversion or submission - albeit under threat of death. Yet once unleashed, wars regarded as holy - rightly or wrongly in terms of such academic criteria as those suggested here -have all too often been taken in practice to license brutality and indeed genocide (in line, sadly, with biblical precedent). The Holocaust is a tragic secular counterpart - burnt offerings in a perverted sacred cause of holy Aryan purity.
Warfare invoking specifically religious reasons as essential to their justification are likely to have occurred among non-Abrahamic religions, but if so they have not gained the global currency of crusade and jihad, and it may be that in such other cases the distinction between holy war and just war is more blurred than in the examples given here. In any case, whichever label is claimed, both are readily available as rationalizations of unscrupulous bellicosity.
On a lighter note, the symbolism of warfare, even holy warfare, may be appropriated for peaceful purposes. In Islam a contrast came to be drawn between warfare as the 'lesser' jihad, and peaceful striving in the path of God (through prayer, preaching, writing, for example) as the 'greater' jihad. In Christianity we find the Salvation Army, where the Lord's battle is being joined against personal dereliction and social deprivation. Its weapons are kindness and generosity and its defence a sense of abiding trust that all will be well - eventually. (crusade; holiness; jihad; Muslim ethics;Christian ethics)