In February 2018, a mysterious alien parasite dubbed the "Chimera" was unleashed on the town of Truth or Consequences, New Mexico. Over the next ten days, the parasite infected the majority of the town's population though government officials were able to successfully contain it for the time being. Those infected by the parasite began to be known as "Roaches". At the request of Aurelia "Six" Arnot, Cohen was the first specialist to arrive onsite to act as an observer and liaison. As the situation worsened, she became the field commander for containment and requested that Six send several Rainbow specialists to provide assistance. Of those specialist, Cohen asked Jordan "Thermite" Trace, Gustave "Doc" Kateb, and Alexsandr "Tachanka" Senaviev to meet her at a command center outside of the city and initiate Operation Chimera.
Thermite and Tachanka arrived at Cohen's tent as she was on a video call with Doc who showed them with a infected severed arm how the parasite mutated to "create monsters". Thermite asked Cohn if it was contained and was told that while it was for the time being, the parasite was adapting faster than they could track. She explained that when they had increased the payload of the ordinance they were using against the parasite, Smashers were created as a result. Cohen then told Thermite that if the parasite broke containment and crossed the border into Mexico, the world would be in for a "long, bad night". Thermite suggested that they nuke the town but Tachanka scoffed at this suggestion. Cohn told him that while it was on the table, they were there to make sure that they keep that as a last resort and that operators were already in the field. She then showed Thermite the information files of Olivier "Lion" Flament and Lera "Finka" Melnikova and stated that it was imperative that they turn the tide in the fight against the parasite.
A short time later, Ash was rehearsing her upcoming briefing to Six concerning a nursery that was built by the Parasite in the town's Sierra Paradise resort. She was interrupted by Thermite who was tinkering with the nanothermite charges that were going to be planted in the nursery. Cohen scolded him as their briefing was in less than a minute though Thermite stated that he only wanted to make sure they were built correctly. Trace then rehearsed his part of the briefing, stating that they would use those charges to burn the nursery as they could not bomb it due to ongoing operations in the area. After convincing Six to approve the mission, they were successful in destroying the nursery.
Cohen later met Doc at the command center to discuss Dr. Ellen Mackintosh who was trapped in the town's hospital. Rainbow was aware that Mackintosh had taken samples of Boyd Brooks' blood following his infection and was trying to develop a cure before the hospital was overrun. This led Doc to theorize that she had been successful in her efforts and that she should be rescued. He then stressed that a cure would allow him to stop working in a tent surrounded with explosives, prompting Cohen to tell him that he could not afford to be a humanitarian in their current situation. Doc then asked Cohen if she had a better option as they were barely containing the outbreak as it was. He also told her that he had overheard talk of detonating a nuclear bomb on the town and hoped she had a better solution. Cohen conceded and approved a mission to rescue Mackintosh. Rainbow was able to save Dr. Mackintosh from the hospital's serology lab and recover her research and Boyd's blood samples.
Some time later, Cohen called Tachanka to her tent. She told him of how the parasite was found by Boyd Brooks in a capsule that had crashed outside of the town and was brought back to his junkyard in hopes of selling it online. Tachanka looked at Brook's online listing and pointed out that the capsule looked to be Soviet. He then realized that he had been bought there due to his Russian nationality. Cohen pointed out that she thought that he would appreciate the heads up and that it would be up to Six to deal with the politics. She then told Tachanka that it was their job to secure the capsule. Through their efforts, they were able to secure the it which allowed Marius "Jäger" Streicher to extract it back to base with an Osprey. Despite their efforts, the town was unable to be saved, though it was contained for the time being.
Over the next few years, the formation of a specialized task force dedicated to containing similar dangers was discussed but momentum weakened despite expert testimony and ongoing T or C quarantine. The reality and proximity of continued Chimera threat was confirmed by Dr. Elena María "Mira" Álvarez who discovered a similar strain of Parasite within Tunguska meteorite. Global consensus followed and REACT was formed with Cohen as its leader.