Bryan Singer had done a great job with X-Men (2000) but escalated it into an even better film with X2: X-Men United (2003).
X2 introduced more mutants to the big screen including Nightcrawler (Alan Cumming) and Lady Deathstrike (Kelly Hu) while raising the stakes in a highly successful follow up. It also explained parts of the first film with well-written dialogue.
The X-Men reluctantly decided to team up with the Brotherhood of Mutants because William Stryker wanted to stop all mutants.
“Mutants. Since the discovery of their existence they have been regarded with fear, suspicion, often hatred. Across the planet, debate rages. Are mutants the next link in the evolutionary chain or simply a new species of humanity fighting for their share of the world? Either way it is a historical fact: Sharing the world has never been humanity’s defining attribute.”