One of the most critical assignments he went on was when his unit (numbering 150 and under the command of Captain Loomis) was tasked to guard a captured Dominion communications array on a planet in the Chin'toka system, that was designated AR-558. He became friends with a young man called Billy, who the Jem'Hadar later killed and decapitated, displaying their grisly prize to Reese. (DS9 short story: "Requital")
Reese remained there for the next seven months, watching as his fellow soldiers died all around him (reducing his unit to 43). He formed a rudimentary friendship with fellow officers Larkin, Vargas and Kellin and made a system of taking one ketracel-white tube from every Jem'Hadar soldier he killed as a necklace (and also took a kar'takin for hand-to-hand combat). Reese also served under Captain Benjamin Sisko, when he arrived with a small squad to help shore up defenses.
Reese survived the final battle with the Jem'Hadar soldiers, but his friends didn't make it. Reese left for the Defiant, while the rest of the surviving soldiers (now numbering 38) left for Starbase 371 on the USS Veracruz. (DS9 episode: "The Siege of AR-558")
After the day was won, Reese returned with Defiant to Deep Space 9. He entered into an alliance with Glinn Ekoor and they plotted to kill the captive Female Changeling as a way to avenge his fallen comrades. Using a Breen isolation suit, Reese and Ekoor infiltrated the signing of the Dominion War Accords. Reese almost assassinated the Founder when she was placed in a holding cell, but was convinced to stand down by Sisko. He then went into counseling to help cope with his anger and grief over the deaths of his comrades. (DS9 short story: "Requital")