X-Men (1963) #405
"Ballroom Blitzkrieg" by writer Joe Casey and artist Sean Phillips. Banshee's X-Corps has declared open warfare on the mutant-hating A.R.M. The X-Men (Nightcrawler, Archangel, Iceman, Stacy X and Chamber) have come to Zurich to suppress the fighting. But Chamber found himself in a strange encounter with Martinique Jason (Wyngarde), the daughter of the malevolent Mastermind, and at the receiving end of an electric shock delivered by Surge. Now Banshee's helicopter gunships have gone missing. All of this, prompts Nightcrawler to demand a thorough tour of Banshee's X-Corps headquarters. He discovers the Scotti! sh mutant leader is using Martinique's psionic powers to keep more malevolent mutants' minds in check in order to get them to work together with his benevolent mutants.
But some "evil" mutants never change their ways. Blob and Avalanche attack Stacy X and the boy named Corbo, attacking with unbridled fury. When Husk finds them two of Banshee's troops dead, the X-Men take charge. But not before Jamie Madrox, the Multiple Man, discovers where the gunships are that Banshee is missing. They are in Paris, and his multiples are piloting them. With the fiery Fever Pitch at his side, he's attacking the City of Lights in the name of mutant pride! Casualties run high as Martinique's mind-control works its evil on Madrox. But to that list, you may be able to add two more - Nightcrawler and Chamber, who get caught on the Blackbird right before it's blown to pieces.