The ARC, short for Armored Response Coalition, is a global alliance of uncorrupted military forces which serves as a plot element in Doom Eternal. It was established by the Allied Nations in 2151 for the sole purpose of combating and containing Hell's invasion of Earth, utilizing Argent-powered technologies to give them an advantage in situations where conventional militaries would be ineffective. Samuel Hayden was appointed Lead Director of the ARC following its formation - overseeing all aspects of ARC weapon and technology development, he repurposed a number of UAC facilities on Earth that had withstood the demonic invasion for weapon development and mass production.
With Hayden's advanced technologies, tactics and use of the demonic Crucible, the ARC was able to slow the invasion for some time but ultimately found themselves fighting a losing battle. A final counter-attack by Hayden, codenamed "Operation Hellbreaker", ultimately ended in failure and almost led to Hayden's destruction. His robotic torso was retrieved by ARC soldiers and placed in the ARC Complex along with the Crucible to be safeguarded by a skeleton crew of scientists.
The ARC was originally headquartered at the ARC Complex, but following its loss they shifted operations to a sea-borne command carrier. Throughout Doom Eternal, the ARC monitors the Doom Slayer's activities and broadcasts news of his progress at each return to the Fortress of Doom.