X-Men #16

Published in January 1966, X-Men #16 was part of the original Silver Age run created by Stan Lee and Jack Kirby. This issue centers on the debut of the Sentinels, the formidable mutant-hunting robots designed by Kirby. The narrative context focuses on the themes of prejudice and technological existentialism that would define the X-Men mythos for decades to come, as the Sentinels were commissioned by anthropologist Bolivar Trask to eliminate the perceived threat posed by the mutant population.

The visual design of the Sentinels by Jack Kirby remains one of the most iconic aesthetic contributions to Marvel Comics. With their massive, colorful, and blocky mechanical forms, the Sentinels provided a stark contrast to the organic superhero designs of the era. The issue was received as a pivotal moment in the series, cementing the X-Men as social pariahs and establishing the Sentinels as the franchise's premier antagonists, a status they have maintained throughout various iterations and reboots.