I agree with the above answer that religion is primarily a surface element in wars; but disagree as to its application to the Crusades.
The Crusades were fought to remove the Muslims from the Holy Land; however it was NOT an effort by the Popes to consolidate power, nor was it because of fear that the Muslims would take Byzantium. Pope Urban II preached the First Crusade after receiving a message from the Emperor of Byzantium asking for assistance; but his real motivation was to hopefully re-unite the Eastern and Western Churches under his pontificate and also to direct the violence of European knights against the presumed enemies of the Church rather than against each other. Rather than "prop up" Byzantium, the Crusaders were a real problem for the Eastern Empire, in fact the Fourth Crusade attacked and sacked Constantinople, and carried away many priceless artifacts. It was the sack of Constantinople in 1204 that rendered it so weak that it could not withstand a Muslim attack in 1453 and fell. There was no concern for a buffer, as there was no attempt by Muslim forces to take Europe after the Battle of Tours some 600 years earlier.
Two quasi-crusades, the Peasants Crusade and the Children's Crusade were primarily based on religion, but both failed miserably and never came close to succeeding.
Another example of "religious" warfare might be the Thirty Years War, which was fought ostensibly as Catholic against Protestant; but was really an attempt by Catholic Powers to control those areas of Europe which had become Protestant. The Holy Roman Emperor, as champion of the Pope, attempted to rid areas controlled by Protestant Princes which would bring those areas back under his own control. So religion is often the surface reason for wars, but not the true reason.
There is no objective answer to this because people can argue about the true causes of wars. However, I would argue that in most "religious" conflict, the religion is at most a supporting factor in the dispute.
Let's look at the Crusades as an example. There were surely religious factors as many Christians thought that it was simply wrong for Muslims to control the Holy Land. However, there were also questions of power and geopolitics. For example, the Europeans were afraid that the Byzantine Empire would fall to the Muslims. This would have meant that there would have been no buffer between the Muslims and Europe. This was worrisome not because of the Muslims' faith, but because they were a strong empire that was trying to expand. So part of the reason for the Crusades were to prop up the Byzantines and protect Europe.
Another aspect to the Crusades was the desire of the popes to assert their authority, not just over the Church but over the rulers of countries. The Church wanted to have more power for itself (not really a religious goal). Sending rulers of countries off on Crusades was a way of uniting Europe, in a sense, under the power of the Pope.
So, as can be seen in this example, religious wars are generally only partly about religion.