A common claim made is that religion has been the cause of the world’s worst atrocities, and that religion has had no greater negative impact than today (or in recent and/or modern history).
(I have heard many Muslims, for example, use this as part of their arguments against Christianity, saying that ever since The Crusades, Christians have been doing nothing but killing each other and killing Muslims. Or they will say so explicitly: not that religion has perpetuated such horrors against humanity but Christianity has. Nothing is said about Islam, however. By definition, anyway, Islam can commit no atrocities. Everything Islam does is justified and just and good and deserved.)
However, as far as recent and/or modern history is concerned, religion has, in fact, not been the main driving force behind violence and wars. In many cases, the driving force was greed for land. (In the era of the Conquistadores, converting to Christianity (specifically Roman Catholicism) had more to do with allegiance to and accepting the yoke of the Spanish/Portuguese crown than necessarily saving the infidels.)
Where ideology or thought played a role, more often than not it was not a religious ideology but a secular one. No greater atrocities were perpetuated against humanity than in the name of pure race and the Fatherland (German nationalist socialism), the working class and class warfare (Soviet Communism, Chinese Communism, Khmer Communism, Nepalese Communism), or ethnic sovereignty (Tamils in Sri Lanka, Sikhs in Punjab, Kashmiris in Kashmir, Protestant Catholics in Northern Ireland, and so on). Thus, looking at things objectively, the claim that religion has been the primary cause of bloodshed is quite false.
Related to this is mischaracterizing martial campaigns. European colonialism was not done to spread Christianity; it was done out of mercantilism. Neither of the World Wars began out of a religious conflict. The Balkan turmoil was an issue of ethnic nation-states – but it just so happened that each ethnicity had also a different religious orientation. The War on Terrorism is not a war between Christianity (must less Evangelical Christianity as is commonly asserted) and Islam but between Western states threatened by Islamic and Islamist militant regimes (whether states or worldwide networks) against said regimes. Some people are all too willing to mischaracterize these conflicts are motivated by or initiated to serve one’s religion, but this is false (insofar as actions by The West are concerned; see next paragraph for an important clarification).
However, having said that, there is one area where religion has been a primary motivator for violence and bloodshed, but this is mainly by the adherents of a certain Middle Eastern religion (and it isn’t Judaism), which makes one to question: if practically every religion has ceased perpetuating or even assenting to violence in its name or for its sake, why does this religion continue to do so? what does this say about this religion’s suitability in the modern world? what does this say about this religion’s values and standards and beliefs and practices vis-a-vis those of other religions? Of course, all of these questions are merely academic. Whatever answer may come about will not change the fact that this one religion has violent tendencies even in the modern world and that it seems these tendencies are intrinsic to it and, thus, cannot be stanched.