The X-Men brought something different to Marvel Comics when the team debuted. Instead of people gaining powers, these were mutants born with their powers and therefore fell prey to the prejudice and hate of a society that can't accept differences. However, the comics failed to make an immediate impact and soon, Marvel stopped printing new issues of the Uncanny X-Men comics.
For a few years, Marvel only reprinted classic issues spread a few months apart. It wasn't until 1975 that Marvel saved the X-Men by completely redesigning the team and printing new stories for the first time since the mid-'60s. Marvel introduced new mutants and created what fans today recognize as the X-Men.
Giant-Size X-Men #1 (July 1975)
The issue that saved the X-Men was Giant-Size X-Men #1 by Len Wein and Dave Cockrum. This might be the most important X-Men issue in comic book history. Marvel had stopped printing stories with the original X-Men for years and it was time to fix what was wrong.
In this issue, five new mutants joined the X-Men to help Cyclops rescue his original teammates. The sentient island Krakoa had captured them and Wolverine, Nightcrawler, Colossus, Storm, Sunfire, and Thunderbird joined up with Cyclops to save them. With this, the All-New, All-Different X-Men were formed.
Uncanny X-Men #94 (August 1975)
While Giant-Size X-Men #1 introduced the new mutants that would form the X-Men, the team had to find its new lineup and that happened in Uncanny X-Men #94 by Chris Claremont, Len Wein, and Dave Cockrum. This was where the team decided who would stay and who would leave.
In the first few pages, several members — new and old — quit the team. Every member of the original squad left except for Cyclops, who remained on as the X-Men's team leader. He then led the new team of mutants into action and Marvel never looked back.
X-Men #100 (August 1976)
In Uncanny X-Men #100 by Chris Claremont and Dave Cockrum, a few big things happened that changed the X-Men forever. In this issue, the team ended up in outer space battling Sentinels. This time the Sentinels were controlled by Stephen Lang, who became one of the team's most vile enemies.
However, the most important moment came at the end of the issue. The team had to find a way back to Earth and there was little chance of them surviving. That is when Jean Grey chose to sacrifice her own life to save them and this was the genesis of Phoenix.
X-Men #101 (October 1976)
Uncanny X-Men#101 saw the first appearance ever of Phoenix. After sacrificing herself in the last issue to save her teammates, Jean Grey came out of the bay where the shuttle crashed as the Phoenix. This was retconned years later when Jean was revealed to remain in a recuperative cocoon in the shuttle and the Phoenix only took her form.
At the time, this issue only revealed that the Phoenix gave Jean new powers and introduced the world to a new cosmic force that proved deadly to all it encountered. This was also the start of the road to the Dark Phoenix Saga.
X-Men #104 (April 1977)
Uncanny X-Men #104 did several things that remain important in the history of Marvel Comics. First up, it brought back Magneto for the first time to battle the new X-Men team. This was Magneto's first appearance in Marvel Comics in three years, where he was turned into an infant in Defenders #16.
Erik the Red reaccelerated Magneto's aging, but to a younger age than he was before he was turned into a baby. This explains why he is younger now than he should be. This was also the first appearance of the Starjammers and it brought the Shi'ar Empire into focus as a major force in X-Men comics.
X-Men #107 (October 1977)
While the main storylines in X-Men comics have featured the team battling hate and prejudice on Earth, there was a period during Chris Claremont's run as the primary author of X-Men comics where they went into the far reaches of space. That really played out first in Uncanny X-Men #107.
The team was transported to a faraway galaxy and the X-Men ended up battling Erik the Red alongside the Starjammers to save Lilandra Neramani, who would later become a powerful ally for Professor X, as well as the Shi'ar race, who would end up tasked with eliminating the Dark Phoenix in a few short years.
X-Men #109 (February 1978)
After a lot of battles in space, the X-Men returned to Earth and met a new threat. This was the first appearance of James Hudson's Vindicator, which meant this was the first official appearance of what would become Alpha Flight.
This issue remains important as it offered up a glimpse of Wolverine's mysterious past, as he was a member of Department H and Canada ordered Hudson to bring him back. Wolverine proved his loyalty here, but this was not the end of this battle.
X-Men #117 (January 1979)
Uncanny X-Men #117 by Chris Claremont and John Byrne did two important things. First, it introduced one of Professor X's greatest enemies in The Shadow King, Amahl Farouk. Second, it showed a bit of Storm's origin as a child pickpocket than Farouk used.
Most of the issue took place in flashbacks with Xavier telling Empress Lilandra of this story from his past at a time where he believed his X-Men were dead. In the end, Xavier agreed to leave with Lilandra and explore the far reaches of space, an important moment in their relationship.
X-Men #121 (May 1979)
Uncanny X-Men #121 was the first appearance of Alpha Flight, the Canadian government's superhero team. Much like when James Hudson attacked and tried to bring in Wolverine before, this time his entire team came along to battle the X-Men.
The two teams were evenly matched and it ended when Wolverine called a truce and agreed to return with the Canadian government to Department H. However, the twist was that Wolverine gave them the slip and snuck back to the United States with the X-Men. This book's artist, John Byrne, created the new Alpha Flight comic series for Marvel after this.
X-Men #125 (Sepember 1979)
Uncanny X-Men #125 by Chris Claremont and John Byrne marked the beginning of the end for Jean Grey. The issue showed how much power she really had as Moira MacTaggert ran tests on her on Muir Island. However, what Jean didn't know was that Jason Wyngarde was manipulating her mind.
This storyline would play in the X-Men's universe in the 1980s with Jean eventually becoming the Black Queen of the Hellfire Club, which would cause her mind to break and the Dark Phoenix to rise. It all started here.