In The Call of the Wild, why is a fight between Buck and Spitz inevitable?

The fight between Buck and Spitz is said to be ‘inevitable’ because Buck wants to challenge Spitz for leadership of the sled team. The narrative explains why:

It was inevitable that the clash for leadership should come. Buck wanted it. He wanted it because it was his nature, because he had been gripped tight by that nameless, incomprehensible pride of the trail and trace—that pride which holds dogs in the toil to the last gasp, which lures them to die joyfully in the harness, and breaks their hearts if they are cut out of the harness. (chapter 3)

Buck, then, takes great pride in his work on the team – the mark of a good and effective sled dog. This makes him ambitious for the leadership. He knows he can achieve the top spot as he is strong enough to fight for it. He is the only dog to seriously rival Spitz as leader as he has greatly progressed in skill, cunning and strength since joining the team.

Therefore, a fight between the two dogs becomes certain. Buck is determined to challenge Spitz and Spitz is equally determined to fight this challenge and maintain his place at the top. When it comes to the crunch, Buck proves himself superior not just in fighting prowess but also mental capacity. He thinks of a way to finally outwit his enemy and it works.

Buck thus proves his credentials as the best dog on the team. His victory over Spitz is the first major step on his way to becoming the most powerful dog of all, as he sheds every last trace of his former domesticated nature to become the wildest of all wild things. By the end of the novel he has, indeed, attained mythic proportions among the local Indians as the fearful Ghost Dog, the undisputed leader of a pack of wolves.

What happened to the team after Buck confronted Spitz in The Call of the Wild?

Buck stands up to Spitz in chapter 3 and defeats him after Spitz attacks him for the third time during a rabbit chase. After defeating Spitz, Buck trots to Spitz's former position as lead dog. However, Perrault and François decide to put Sol-leks into the coveted position, which upsets Buck. Buck knows that he has earned the position of lead dog and pounces towards Sol-leks several times. Eventually, Perrault and François relent and allow Buck to lead the sled. Buck excels as a lead dog and is by far the most talented lead dog Perrault and François have ever seen. Buck's leadership skills are unmatched, and he motivates the other dogs to push themselves like they've never had before. Buck punishes Joe, forces Pike to give maximum effort, and the entire group recovers its solidarity with Buck as their leader. Buck is so effective as a lead dog that his team breaks a record for the fastest run from Thirty Mile River to Skaguay.

What incident interrupted Buck and Spitz's first fight in "The Call of the Wild"?

While Buck and Spitz are fighting, the camp is suddenly overrun by a pack of "starving huskies, four or five score of them, who had scented the camp from some Indian village".  Mad with hunger, the huskies steal all the grub they can find, and when they are confronted by "the astonished team-dogs", set upon them with a vengeance.  There is no opposing the skeletal scavengers, who, "with blazing eyes and slavered fangs", attack with terrifying and desperate intensity.  Buck must fight for his life, and at the same time, fend off treacherous attacks by Spitz, who, despite the chaos, still singlemindedly pursues their battle for ascendancy.  Battered and bleeding, the team-dogs eventually flee, and gather together later in the shelter of the forest.  The personal rivalry between Buck and Spitz is finally put aside for the present, and at daybreak, the team "limp(s) warily back to camp to find the mauraders gone" (Chapter 3). 

What action does Spitz take against Buck during the attack in The Call of the Wild?

Spitz deeply resents Buck, seeing him as a threat to his dominant position in the pack. Buck is a strong, hardworking dog, so it's not hard to see why Spitz should come to regard him as such a dangerous rival. One night, when Buck goes to try and get some food, he finds his space occupied by Spitz. Tensions have been building between the two alpha dogs for some time, and as they circle each other threateningly, it seems that a full-blown fight is imminent.

But just when it looks like the fur's about ready to fly, the camp is suddenly invaded by a pack of half-starved huskies, crazed with hunger. Three of them attack Buck, wounding him badly, but not before he sinks his own fangs into one of their jugulars. As if this isn't bad enough, Buck's also being attacked by the treacherous Spitz, shamelessly stealing an opportunity amidst all the confusion to grab him by the throat. This shows just how much Spitz really hates Buck. Instead of standing alongside his fellow trail dog to see off the pack of wild, crazy huskies, he uses their vicious attack as a cover for attacking his rival.

Why does Spitz bully Buck in The Call of the Wild?

In some way you answered the question yourself.  Spitz was a bully.  He bullied Buck because that was his nature.  Spitz was the lead dog.  It was his job to lead the team.  Buck was a newcomer to the team and Spitz tried to treat him as he had treated all the other dogs.  Just like the bully at school, he tried to push Buck around, take his food, and show him who was boss.  Buck wasn't going to have it.  He was not going to be bullied so he finally fought back and replaced Spitz.  The difference was that Buck led by example and earned the respect of the team.  Spitz did not earn their respect, he just intimidated them into compliance.

"Spitz is the lead-dog of the team, and he is Buck's nemesis. Buck resents his power and intends to challenge him, knowing that it must be a battle to the death. When Buck, in the full frenzy and "ecstasy" of the "blood lust," closes in on a rabbit, and Spitz steps in to claim the prey for his own, Buck attacks Spitz. After a long and bloody fight, Buck is the victor, the "dominant primordial beast."

What two actions does Buck take to usurp Spitz's lead position in The Call of the Wild?

I think you're talking about things Buck does to challenge Spitz's authority before they have their fight to the death. Buck becomes lead dog after he kills Spitz.

Buck deliberately interferes when Spitz tries to punish one of the other dogs. One morning Sptiz couldn't find Pike after a heavy snowfall, and he was furious with Pike. Once he found him, Spitz went to punish Pike, and Buck flew in between them. Both Buck and Pike attacked Spitz, but Francois stopped Buck with his whip, and Spitz was able to punish Pike.

For the rest of the trip, Buck interfered with Spitz, but he did it when Francois wasn't around because he remembered the sting of the whip and lash. Buck caused all of the other dogs except Dave and Sol-leks to rebel against Spitz so that the team no longer worked together. There was always fighting among them, and Francois had to spend extra time trying to keep the peace until they got to Dawson.

What caused the final battle between Buck and Spitz in The Call of the Wild, and what was the outcome?

The dogs do bond a bit when they are in the sled, and in some ways they get stronger.  However, they spend most of their time fighting.  Buck, for instance, has to fight to the death (the other dog's death) in order to remain the alpha, lead, dog.  The lead dog is really responsible for ensuring that all the dogs get along.

What caused the final battle between Buck and Spitz in The Call of the Wild, and what was the outcome?

The dogs had to go on the trip to deliver mail because if they didn't, they would have been beaten to death or shot.  This was not a pleasure trip for anyone - the people who owned the dogs forced them to make this mail run because that was how they made their living.

Here is a link to the eNotes' summary:

http://www.enotes.com/call-wild/summary