I'm fairly certain humanity would find plenty of reasons to wage war if religions were not around to blame it on.
Governments would, of course, but it would be much more difficult for them to convince soldiers to actually go fighting.
Example 1, Operation Iraqi Liberty: US government wants to gain control of resources in Iraq, but they tell their people it is really for ideals of freedom and to do God's bidding. Iraqi government wants to stay in power and keep oppressing their people, but tells their people they need to fight because God is the greatest.
Example 2, US Civil War. North wants to instate an economy of small farmers who can buy products of northern industry, but tells their people that it is because we're all brothers (which may very well be true, but I don't see the same people so eager to go to war when there is no money to be made); the South wanted to keep mooching off slave labor, but said that the Bible advocates slavery, so it's really a holy war.
In all cases, it is difficult to convince someone to risk taking a bullet if you cannot convince them that there is life after death, and that is eternal and so much better than this valley of tears. This creates a problem for the few countries that disavowed religion and were involved in wars, notably the Soviet Union, which had therefore to develop its own pseudo-religion of Rodina and, just to be on the safe side, extensive usage of barrier troops.