In Paulo Coelho's The Alchemist, the purpose of Santiago's involvement in the tribal wars (and the reason that the alchemist seems to abandon him) is that Santiago must prove to himself—not to the soldiers or the alchemist—that he is capable of mastering all that he has learned. Until now, Santiago has learned to look for omens along the way and to listen to his heart and the language of the world. He has discovered important things from those he meets, e.g., the crystal merchant. He has found out that some men's  "knowledge" (the Englishman) is not worth knowing.

When the alchemist brags to the chieftain about Santiago's powerful gifts, Santiago's first reaction is horror and disbelief—not only for what the alchemist claims, but for what Santiago sees as impossible to achieve on his own part. He has no confidence because he has no sense of his own power. Santiago believes in the power of the alchemist. The alchemist tells the young man that rarely do others believe in your powers:

When you possess great treasures within you, and try to tell other of them, seldom are you believed.

We see this soon after: the alchemist speaks of his "apprentice's" powers, but the chief and his men simply laugh. Ironically, Santiago doesn't believe in himself either! But the alchemist knows how far along the young man has come, and that he is ready to prove himself.

Santiago makes it clear that he is sure he is going to die, for he has never done any of the things the alchemist alludes to in his discussion with chief. Santiago is terribly afraid.

The boy was too frightened to listen to words of wisdom. He had no idea how he was going to transform himself into the wind. He wasn't an alchemist!

Fear can be a powerful motivator, and fear of death motivates like nothing else. In three days, Santiago is brought before the chief to prove that the alchemist's words are true. Facing the possibility of death, Santiago uses all he knows to make the wind move him, just as the alchemist had said. The old man expresses his belief in Santiago's ability to perform:

If a person is living out his Personal Legend, he knows everything he needs to know. There is only one thing that makes a dream impossible to achieve: the fear of failure.

In this process, Santiago learns more about the world, much more about himself, and he even teaches things to the "world"—to the desert; he speaks to the wind and the sun. Ultimately, the boy "controls" the wind, as the alchemist said he could.

While speaking to the sun, Santiago expresses the essence of what is occurring to him during this test, though he doesn't realize it:

...each thing has to transform itself into something better...

Santiago does just this—his transformation comes when he gets the wind to move him. He is able to fulfill the tribal chief's wish, and proves to himself that he is capable.

"The darkest hour of the night came just before the dawn" is a similar to the saying by English theologian and historian, Thomas Fuller. In this instance, Santiago faces what may be his last moment of life if he cannot master the wind. However, at this particular instance—when his prospects for staying alive seem so slim, the darkness represents the most desperate moment for the boy—his death. However, this swiftly changes. In proving himself, it is as if the sun starts to rise; his success is that much greater because things had seemed so bleak a short time before.

Why did Santiago have to go through the danger of tribal wars on the outskirts of the oasis in order to reach the pyramids in The Alchemist?

Santiago needs to face many trials as he undergoes the hero’s journey to find his Personal Legend.

Santiago’s quest is a version of what we call the hero’s journey, a mission that an ordinary person finds himself on when he least expects it.  The hero’s journey is an archetypal quest that is a megamyth or monomyth, a myth found across many different cultures throughout time.  Santiago’s quest to find his Personal Legend and the treasure is very similar.

In the hero’s journey, the person is often looking for treasure.  The hero, usually a young boy, often is very ordinary at the outset.  In this case, Santiago was a shepherd.  The hero then gets some kind of call to adventure, in the case a dream, that causes the hero to go on the journey.  Along the way, the hero will meet some predictable stages that are part of the journey.  One of those stages is to meet an oracle.  In Santiago’s case, that oracle is the alchemist.  Also typical to the journey is the trials the hero must undergo before he can find his treasure or, as he is really doing, accomplish his task and become a hero.

The description of the tribal wars, as Santiago thinks about them, make it clear he is aware that they are just some of the trials he needs to overcome in order to find the person he loves, discover his Personal Legend, and get his treasure.

There was the desert that he had wandered for so many months; despite all that time, he knew only a small part of it. Within that small part, he had found an Englishman, caravans, tribal wars, and an oasis with fifty thousand palm trees and three hundred wells. (Part II)

So you can see, trials are just part of life.  They are part of the hero’s journey.  They are the barriers that a hero must overcome in order to gather the material and immaterial things that he needs to reach his goals.  A hero needs wisdom and bravery.  If he never faced any barriers, how would he ever get them?  Things can’t be easy.  There have to be challenges, because we have to know that we are worthy, and we have to face the little challenges and defeat them before we can face the big ones, so that we know we have it in us to face those big ones.

In The Alchemist by Coelho, why did Santiago have to experience the dangers of the tribal wars in order to reach the Pyramids?

In Paulo Coelho's story, The Alchemist, it is only because of the news of the tribal wars approaching that Santiago sees the vision of the hawks fighting. When he speaks of what he has seen, the alchemist comes to find out who is reading signs based upon the behavior of his birds. It is here that Santiago and the alchemist meet, and the wisdom of Santiago's responses based upon what he has learned on his long journey, impress the alchemist. They begin their acquaintance. The two start out on a journey across the desert.

When the alchemist and Santiago reach one of the warring tribes, they are taken prisoner, accused of spying. As the alchemist explains why they are there, he identifies the boy as an alchemist who can control the wind. And the alchemist says that after three days, if Santiago cannot do so, they will forfeit their lives.

Santiago is desperate: he has no idea how to do what the alchemist has promised. However, when the time comes, he joins himself with nature—with the wind itself. Then the wind obscures the sun so that the boy can address it. Between the wind and the sun, the boy is able to "reach through to the Soul of the World," which was a "part of the Soul of God," and learns that the Soul of God was a part of him...and he knew he could perform the miracle that the tribal chief expected of him. He does so, and finally they are released.

When the boy and the alchemist separate, the boy heads to the Pyramids, where he is robbed and beaten, but told of a recurring dream one of the thieves has had about Santiago's home in Spain. At this moment he realizes what he must do: go home to Spain. If he had not experienced the dangers of the tribal war, he would not have reached the Pyramids and made his discovery.

Wherever your heart is, that is where you'll find your treasure.