Avengers vs X-Men

Avengers faction
Captain America (Steve Rogers): Tactical commander who seeks to stop the Phoenix before it touches Earth.
Iron Man (Tony Stark): In charge of technological development to analyze and destroy the cosmic entity.
Thor: Lead the space brigade sent to intercept the Phoenix in outer space.
Scarlet Witch (Wanda Maximoff): Her chaos magic turns out to be the only force capable of harming the Phoenix avatars.
Spider-Man: He fulfills a fundamental moral role and plays a key heroic role in K'un-Lun.
Other members: Black Panther, Hawkeye, Red Hulk, Black Widow, Luke Cage, Ms. Marvel (Carol Danvers), Spider-Woman and Doctor Strange.

X-Men faction
Cyclops (Scott Summers): Supreme mutant commander, willing to risk it all
all for the rebirth of their species.
Emma Frost: Strategic telepathic support and co-leader of the mutant cause.
Namor: King of Atlantis who escalates the war by directly attacking the nation of Wakanda.
Colossus and Magik: Siblings with enormous offensive power who are part of the main battlefront.
Hope Summers: The young "Mutant Messiah", center of the dispute between both sides.
Other members: Magneto, Gambit, Psylocke, Sunspot, Danger, Rachel Grey, and Emma Frost.

The first confrontation erupted on the beaches of Utopia Island when Captain America traveled there personally to ask Cyclops to hand Hope Summers over to the Avengers' custody.
The negotiation: Steve Rogers tried to reason with Scott Summers, explaining that the Phoenix Force was a destructive threat and that Hope needed to be kept safe in a secure location under supervision.
The first shot: When Steve hinted that he had backup to secure the girl, tension boiled over: Cyclops responded by firing a powerful optic blast at his chest that sent Captain America into the water.
Beach landing: Immediately after the attack, Captain America gave the order for the other Avengers to intervene.

The main conflict
The trigger: The Phoenix Force returns to Earth seeking Hope Summers as a host, the first mutant born after the mass extermination of M-Day.
Avengers' stance: Led by Captain America, they see the Phoenix as a cosmic-level threat capable of destroying the planet and demand that Hope be taken into custody.
X-Men's stance: Led by Cyclops, they see the entity as the last chance of salvation to restore the mutant race and argue that Hope must fulfill her destiny.
The Phoenix Five
During the decisive battle on the Moon, Iron Man attempts to destroy the Phoenix Force with a special cannon. The entity doesn't die, but instead splits into five parts and possesses the main mutants:
Members: Cyclops, Emma Frost, Namor, Colossus and Magik.