I think it's money that causes most war. As much as I hates me some theists, I can't allow myself to be so naive as to believe that people generally act on ideals rather than greed.
Think about it. What is the one thing we get out of wars? We get territory, power, and resources which pretty much translate directly into money. The Romans built their empire not through religious desires, but through ambition for power and wealth. The Mongols didn't have a priest exhorting them to rape and pillage, but they just wanted something to you know, rape and pillage. Why did the Spanish exterminate the Mesoamericans? Religion was only a front for that and they knew it. They just wanted gold.
The American Revolution and Civil War were ignited due to differing opinions on how money should be made and distributed. In the former, the colonists wanted economic independence and in the latter, the South wanted cheap labor.
While political differences do incite some wars (WWI for example), those ideologies are driven by the quest for power which in modern times is quantified in money. Why did Hitler want a new Reich? Because the German economy had been raped after WWI and war seemed like a great way to get the people pumped up. Sure there were some bits about the Jews and Commies but remember why he hated them, they were a threat to the people's wealth.
The United States didn't have to get involved in either World War, but I am of the opinion that the banking establishments were directly responsible for US involvement. Face it, there are plenty of chances for profiteering in war. For instance: The United States government is forced to borrow funding from the Federal Reserve. This is done at an interest rate, thus netting the semi-private bank a profit. In wars, greater and greater resources are required to fund military campaigns, therefore producing greater profit.
Now, take a look at the current conflict in Iraq. While they may or may not have possessed WMDs at the time of the invasion, it was a pre-emptive strike. This wasn't done for the idealistic reasons of "spreading democracy", no it was at best an attempt to prevent damage to US property and at worst just another excuse for war profiteering.
Don't get me wrong, I can't stand organized religion, but it is the will to power that motivates people's worst actions. Sure, there's some bearded man with a Koran plotting my death right now, but I think that you could placate him should you just thrust a pile of money in his direction.