The storms rolls in bringing with it life giving rain but also winds and floods which destroy homes and sweep away loved ones. So it is that storms are the most confusing of natural phenomenon - needed, even desired - yet at the same time, feared. Nearly everyone has experienced fear as a storm rolls in, the sense of dread as the sky turns dark andthe winds howl outside like demons. Then theirs a flash of lightening followed by a boom. But to the Indo-Europeans this boom, this lighting was an ally, brought to defeat the evil, to defeat the monsters which they feared. Of all the deities of the Indo-Europeans the most commonly attested is the lord of thunder and the storm. He is the warrior who banished the darkness and destroys evil with his thunder bolts.
This god of thunder of the storm was the chief deity Zeus in Greek legend, the Chief Deity Indra of the Rigveda in Indian mythology, as well as Perun the most likely chief deity of the Slavic peoples and one of the most important deities in Celtic, Scandinavian, Hittite and Germanic folklore.
He is the only deity I know so commonly attested to as being of primary importance in all Indo-European faiths which we still have a record of. That he is so commonly attested to can be taken one of two ways, the first is that he was the lead deity of the Proto-Indo-European peoples. Certainly he appears to be a good candidate for this and it seems very likely that he was extremely important. On the other hand we should expect that the deity best attested to would be a defender of the people against strange demons for all nearly Indo-Europeans migrated to strange lands. The people who wrote the Veda, the people who entered Europe were thousand of miles from home, for what ever reason they had traveled across the world into the unknown where only the sky and the storms remained the same. The peaceful sky however could not protect them, it could not defeat the demons they feared, only the storm could do that. Only the axe, arrows, or hammer of the gods could strike down the giants of the new land in which they found themselves. So while one can presume that the Indo-European’s thunder god is yet a sign that they were war like, (which they likely were were) one can also take it as a sign that they were afraid, that they needed a guardian in the sky. Certainly when one considers what his original role likely was this idea makes some sense. Consider after all why it is that the thunder is considered to be the weapon of good? Most people who sit in the darkness with the storm raging around them would not consider this to be so, but remember that religion has its roots in function. When a weeping fearful child climbs into bed with their parents as the storm rages around them and asks what the thunder is what are the parents to say? What story do they tell their baby to comfort them? It seems unlikely that the story of the Proto-Indo-European parents would have told their child would have been one of demons haunting the world, for there were enough of those in the darkness and the storm. What was needed was a sign of something trying to defeat the fears of the child and so they would tell their child of how the oak tree which they had made offerings to, the spirit of the oak tree which supplies some of the food was out in the storm fighting the demons, fighting the darkness which the child feared. In this way the child would know that the thunder was his friend and so would be comforted.
So while the god of the oak trees and the thunder may be a god of war, making a warrior one of the most important gods to the Indo-European peoples this may only be because a warrior is sometimes what is needed to give comfort not just to those going into battle but also to those who are weak and afraid.