well, one, i think that peace and agreement are two very different things... and the agreement to disagree over religion is a good start. but i still don't feel like religion is typically a central issue - religion is more often one of those things that's generally used, like a tool, to fight in a power struggle - like people using the bible to argue about slavery, racism, sexism, and sexual orientation.
this. a balance of power is good, though preferably not with the pervasive fear that characterized the cold war.More effective than conquering the rest is having a balance of power, where it is too risky for any state to wage war against another. As a very loose example think the USA vs USSR. Both were afraid of retaliation against a strike, in addition to the excruciating pain of the attrition of war.
i don't know if that's possible, but for right now it's the most ideal but still realistic situation i can imagine.