The Battle of Rell was one of the first and largest full-scale engagements between the UNSC and the Insurrectionist forces of the Legion. This battle was also the first engagement that the SPARTAN-IV Yankee Company participated in, alongside a team of specialists from X-Ray company and a company of Marines. The SPARTAN-IV's were deployed near a ridge in the southern hemisphere of the planet, near a large rainforest that served as the Rebel command centre. Yankee Company, whom had recently finished training, were to be dropped in to secure the outer defences: A wide array of trenches, guard towers and AA guns, so that friendly air support could move in and destroy the base.
Using SOIEV's, the Spartans inserted close to the trenches. Wulf Team was deployed along the eastern side of the ridge, while Yankee Company secured he outlying bases. Not long after, the entire trench line was secured, with n casualties. Meanwhile, on the other side of the rainforest, the 25th Marine Expeditionary Force had engaged the bulk off the Legion forces. Upon finding out that their outlying outposts had been captured, the Legion launched a counterattack. The Spartans defended the ridge from wave after wave of Legion attacks, holding the line after several bombing runs by rebel ships. After almost an hour of fighting, UNSC HIGHCOM ordered the Spartans to fall back, evacuating them in Pelican dropships. The 25th was also pulled back, despite having moved into Legion territory. As they fell back, they took heavy casualties, with over 2/3 of their numbers killed.
With friendly forces evacuated, the UNSC Romulus fired several MAC rounds, followed by heavy Longsword bombing to completely destroy the base. The Battle of Rell was largely seen as a test of Yankee Company in combat, though ONI and the UNSC did not expect the Legion to use augmented soldiers in battle. The company lost 39 Spartans, and many more were injured, including Lysander-Y050, who required a robotic prosthetic arm, and Ross-X090 of Wulf Team, who sustained heavy burns due to a malfunctioning incendiary grenade.
After tests on the bodies of enemy combatants were carried out, it was confirmed that they had all taken a dose of 009762-OO. In response to this, efforts were diverted to finding the drug factory where it was being made. Wulf Team was sent to the Pegasi Delta in search of the factory. The loss of thirty-nine members of Yankee Company on their first engagement was seen as a major embarrassment, especially after their years of intense training and the cost of each individual supersoldier. This also marked the first time that the Legion had engaged UNSC forces in a pitched battle rather than through a raid or covert bombing, raising concerns that their foe had gained a great deal of strength over the years. It was later discovered that the Legion was using Rell as a staging post and had committed a great deal of their resources to the planet; this made the UNSC's success in the battle a major blow towards the rebel army.