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The Attack on White Forest was a raid in force by Combine forces which had withdrawn from City 17 prior to its destruction, against the Rebel held base of White Forest. The Combine's primary objective was to destroy the guidance array and shoot down the White Forest rocket, which contained a satellite capable of reversing the effects of the Combine superportal. Closure of the portal would mean permanent isolation for the Combine on Earth. The failure of the Combine to both capture the base and destroy the satellite before launch was a mortal blow to their forces on Earth, who were now isolated. The Rebel victory however, was bittersweet, as Eli Vance was killed shortly after the attack ended. Or was it, as a mysterious force "somehow" saved him, but instead Alyx was somehow nowhere at sight, despite both Eli and Gordon clearly remembering her with them, as they're planning to find the Borealis and Judith, all together.
Following the destruction of City 17, the rebels had evacuated their forces and citizens to the countryside, using White Forest as a rallying point. With the Combine in retreat, and having suffered a catastrophic defeat in City 17, the Resistance could now put their plan of using the old Xenian resonators in Earth orbit to seal the portal storms, and the infant superportal developing above the ruins of the Citadel, into action - something previously unimaginable while the Combine was in such a strong position.
Following their defeat in the city, the Combine began a strategic withdrawal, however, after realising what the Rebels were planning, a substantial force was hastily assembled and ordered to raid White Forest. Meanwhile, Alyx and Freeman had been attempting to reach White Forest by road, and skirmished with some Hunters, before reaching the White Forest Inn. There, they were cut off and attacked by a substantial force of Combine troops, aided by hunters, however, the pair managed to hold their ground. They continued, until they reached the perimeter of White Forest. There, they encountered a single strider from the wreckage of a crashed Combine Dropship, which was promptly defeated by Dog.
Initial Combine maneuvers and final attack
White Forest was subject to a series of false alarms, due to crows nesting where the perimeter sensors were located. As a result, the rebels often let down their guard, which allowed a force of Hunters and a detachment of Overwatch troops to infiltrate the base in force, with the intention of reaching the silo, and destroying the satellite before it could launch. This attack was eventually beaten back, prompting the Combine to go with their second plan - an attack in force by a large detachment of striders.
Instructed in the use of the prototype Magnusson Device, Freeman and the troops at White Forest prepared for the onslaught. The final attack consisted of at least a dozen striders, replete with at least two to three hunters providing escort duties to each strider. While this overwhelming force was capable of inflicting heavy damage on the rebel's outlying defensive posts, the use of the Magnusson Device proved invaluable, and the attack was repulsed.
Following the failure of the raid, the Resistance could now launch their satellite into orbit, and was successful in neutralising the Combine superportal, completely isolating their forces on Earth, with no chance of rescue or reinforcement. The Combine struck a moral and psychological blow to the Rebels, however, as after the battle ended, a pair of two Combine advisors ambushed Alyx, Eli, and Freeman in an aircraft hangar, killing Eli, one of the most important and revered figureheads of the rebels.
Mere moments later, his death was somehow prevented through the intervention of the G-Man, who sent a younger Alyx into the future to witness her father's death and granted her the power of Vortigaunt energy to kill the Advisor before it could kill Eli. This resulted in a new alternate timeline where Eli lives but Alyx disappears. A furious Eli swears revenge on the G-Man. Dog arrives with Gordon's crowbar, which Eli grabs and hands over to Gordon.