X-Men: The Magneto War #1

The battle that will consume mutant and humankind alike--Joseph vs. Magneto! The Master of Magnetism is back, his thirst for power greater than ever! On the brink of unleashing his most ambitious and deadly plan to date, he must first destroy the one obstacle that could impede his path to domination--his enigmatic twin, Joseph! Simultaneously, the X-Men learn that their onetime teammate, whom they'd thought to be the one and the same with Magneto, is a greater mystery than ever! And those whose lives have been impacted by the two--Professor X, Wolverine, Rogue and Gambit--realize that they will soon be confronted with decisions that could very well tear the team apart! It's the beginning of a stunning saga that will drastically alter the status quo of mutantkind in the Marvel Universe!

This story continues in Uncanny X-Men (Vol. 1) #366.