Bastion was "born" when the giant robotic Sentinel Master Mold-having absorbed the advanced Sentinel prototype Nimrod into its systems-fought the X-Men, who saw no other way to stop the virtually indestructible amalgam except to force it through the Siege Perilous, a pan-dimensional portal which subjects those who pass through to the judgment of the universe's higher powers. Reemerging from the portal as an amnesiac human/Sentinel hybrid with a seemingly immortal essence, Bastion's prime directives were soon reawakened, and he began assembling the international anti-mutant strike force Operation: Zero Tolerance (OZT). Bastion confined the X-Men's founder Professor Charles Xavier in an OZT facility along with the artificially-created mutant Mannites, and then captured young former X-Man Jubilee. Soon after, Bastion put OZT into full effect, utilizing Prime Sentinels - a new breed created from humans unknowingly implanted with nanotechnology - to attack the X-Men, quickly capturing several members. Bastion took control of the Xavier Institute and accessed Xavier's files, despite opposition from Cable and Caliban of X-Force. The five captured X-Men and Jubilee ultimately managed to escape, and OZT was shut down by the U.S. government. Bastion was remanded into S.H.I.E.L.D. custody, but he escaped and returned to the former OZT facility. There, he absorbed the energy from the Master Mold unit with which he had created the Prime Sentinels and sought to transform Machine Man into the Sentinel Supreme; he was opposed by Cable, and seemingly destroyed.
Bastion survived and was recaptured by S.H.I.E.L.D. An attack by Apocalypse's Horseman Death (actually a brainwashed Wolverine), who was seeking to kill the Mannites, left only Bastion's head still intact; Bastion survived. Later, Bastion's head was recovered by the robotic Gatekeeper servants of the techno-organic being Mainspring. Bastion co-opted one of the Gatekeeper's bodies, and used it to attack the techno-organic hero Warlock and his allies. Defeated, Bastion's head was recovered by Mainspring, who stripped away the layers of his programming to reveal the original Master Mold's source code and, using the alien transmode virus he was infected with, rewrote Bastion's programming, transforming him into Template. The virus soon took complete control of Template and forced him to construct a Babel Spire on Earth to signal the alien Technarchy. To oppose them, Mainspring destroyed himself along with Template, while Warlock managed to destroy the Babel Spire.
Bastion's head was later retrieved by U.S. government agents in order to access the information he had stolen from Xavier's files. The information was retrieved by the X-Men, but not before Bastion attempted to sow mistrust among them by confronting them with misleading holographic scenarios. Recently, the Purifiers reformed Bastion.