People always wanted someone else's resources, or the dick at the top wanted more power and glory, or, really, whatever.
That really applies to the crusades too. Sure, religion made a fine tool to motivate the plebs to go get killed in someone else's war, but the ambitions behind it were power, glory, influence, resources, etc. (A western kingdom in the middle east bypassed Venice as a middle man in the land trade with the orient, for example.)
The first crusade basically started when the Byzantines asked for help with pushing the Seljuk Turks back from their teritory, and got a bit out of control when the Pope (another prick in a privileged position) took it as an oportunity to extend his own hegemony. There was nothing fundamentally religious about that.
Even other so-called religious conflicts often actually had nationalistic reasons. The Hussite Wars which left whole counties in Germany in smoldering ruins, were driven by anti-German sentiment and religion just made a convenient pretext. The thirty years war was largely a grab for power, and not just by the German princes involved: France, a catholic nation, actually bankrolled the protestants and Sweden to intervene on the protestant side, just to weaken Germany. Etc.
And again, bloody wars happened without it too.
The Mongols under Gengis Khan ravaged half of Asia and Europe, and spread terror by massacring and enslaving anyone who didn't surrender, to an extent that makes even all the crusades put together look like amateur work.
Then Timur Leng (a.k.a., Tamerlane) did the same all over again, except from the South, dealing a nearly fatal blow to the Ottoman Empire.
The Turks themselves had gotten there by ploughing through Persia, the Caliphate, and the Byzantine empire, just because they could. There was no religious intolerance needed to attack someone.
China, I already mentioned.
The Franks ploughed through a whole Europe and for a short while made an empire actually bigger than the Roman one. Most of that happened before Charlemagne got christened, so...
And even when you look at Roman times... while nowadays we look at an "age of migrations" and think only then they happened, they actually happened all the time. Rome just happened to be perfectly capable of stopping them before AD times. The only thing different in the "age of migrations" was that this time they were capable of migrating right through Roman defenses.
Caesar started his conquest of Gaul by answering the call for help of the Allobroges, who were an allied Gallic nation, and in danger of being ploughed through by the migrating Helvetii.
The Teutones and Cimbri ploughed through the whole Europe and initially spanked Rome, at the time already embroiled in another war, so hard that it no longer had any recruits that could afford their own equipment. That's what made the Marian reforms necessary.
And so on, and so forth. There was _always_ someone showing up at the borders and trying to move through, or at least raid the estates on the Roman side. See the Dacians for some who were actually damn good at coming over and plundering regularly, before Trajan finally spanked them.
And before that, just look at Mesopotamia, where they fought each other all the time, in spite of recognizing each other's deity.
But generally, I get the feeling that people look at ancient history through the glasses of modern day. You know, expecting people to be mostly peaceful and nice to each other, until offered a reason about on par with 9/11, or at least a crusade or jihad.
In reality, war was a constant.
E.g., in Rome, a nation that prided itself on never starting a war (ok, so they lied a little
), the doors of the Temple Of Janus were supposed to be closed in times of peace and open in times of war. They were only closed twice during the republic, and three more times during the empire. That's how constant war was.
Not being an intollerant monotheism, didn't do much to stop that constant warfare.
) 