- "The time for diplomacy has passed, the Helghast do not see reason, they started this war when they invaded Vekta, and it's up to us to conclude it."
- —ISA spokesman, to UCBN News[src]
Operation Archangel was ISA Alpha Centauri's counter-invasion of Helghan in retaliation for the Helghan Empire's invasion of Vekta during the Second Extrasolar War. The objective was to quickly end the war by capturing Autarch Scolar Visari and replacing the Helghast regime with an easily controlled government. However, the ISA were underestimated by the tenacity of the Helghast and failed to accomplish their goals, costing nearly the entire loss of their invasion force in the nuclear destruction of Pyrrhus, and Visari was killed and in turn prompting the Helghast First Army to destroy what is left of the ISA invasion force.
The ISA intended to defeat the Helghan Empire by arresting Autarch Scolar Visari and wrestling away control of Helghan, before establishing an easily controlled ISA-puppet regime to govern the planet, thus stopping the hatred and militarism that was the hallmark of Scolar Visari's reign. The plan was first put forward by the United Colonial Nations, in cooperation with the Vektan Council, to end the war and destroy the Helghan Empire. However, the UCN soon turned against the plan, preferring to isolate Helghan instead and keep Visari's forces bottled up with a significant force of blockading ships in orbit.
The Vektans; however, were utterly unwilling to see the Helghast go unpunished for the atrocities they committed on their world, and claimed that so long as Helghan remained unpacified, the whole of humanity would be threatened. The operation consisted of three waves of assault with a total of 12 ISA battle groups. The first wave was to secure a local firebase and capture the Helghast capital of Pyrrhus.
The second wave in which Battle Group Mandrake took part, had its primary objectives to capture Scolar Visari and eliminate the ground opposition as well as the remaining ships and orbital defenses. The third and final wave was in charge of deploying supporting troops to assist the ISA ground forces, aid the other battle groups and escort Battle Group Mandrake in their return to Vekta with Visari as a prisoner of war to be judge for his war crimes. By 2359 the ISA put the plan into effect, acting against the mandate of the UCN whom continue to aid ISA efforts.
Under the command of now Colonel Jan Templar, the invasion soon failed as the ISA forces sorely underestimated the sheer scale of the Helghan defenses and resolve, as well as failing to take into account the charismatic leadership of Scolar Visari and the fanatical Colonel Mael Radec.
Upon their arrival to Helghan, the ISA Navy engaged against the Helghast's orbital fleet and quickly establishing a total orbital blockade. However, the ISA suffered heavy losses from anti-orbital Arc Cannons and causing a temporarily setback to the invasion. ISA-sponsored mercenaries were sent planetside to disable the cannons, allowing the ISA to deploy their troops to Pyrrhus while ISA ships bombard the city as cover for the ground assault. Despite facing stiff resistance from the Helghast, the ISA were able to capture Visari Square and the Helghan Military Academy. Just as the ISA prepare to attack the Imperial Palace, Colonel Mael Radec activates a network of arc towers hidden beneath Pyrrhus, killing hundreds of ISA soldiers and breaking their momentum.
The siege of Pyrrhus was reduced into a brutal urban war, with both sides taking large amounts of casualties. Colonel Radec launched an attack on Battle Group Mandrake, boarding or destroying several ships including the New Sun, where Radec killed Templar and extracted the launch codes to nuclear weapons that were stolen from the ISA two years prior on Vekta. A dying Templar maneuvered the New Sun to crash directly into the Tharsis Refinery petrusite grid, causing to disrupt the Helghast's arc network. While Templar's actions allowed the ISA an opening to Visari Palace, Visari detonated a nuclear warhead over Pyrrhus, killing most of the ISA forces and city's population. Prior to the detonation, Radec had ordered the Helghast Army's senior officers and their forces to move out of the blast zone within ten kilometers from the Corinth River while sacrificing their forces that remained in the city in order from alerting the ISA to the nuclear weapons' deployment.
The remaining ISA forces fell under the command of Captain Jason Narville, who managed to avoid the nuclear explosion entirely, due to being situated outside the capital city in the Helghan desert wastes while conducting rescue operations. This small contingent then gathered any surviving troops and launched a desperate, all-out attack on Visari's Palace and capture the dictator.
After a brutal battle between Visari's Palace Guard and the remaining ISA forces, the Helghast forces were driven back to the massive entrance of the Palace where they fought, and died, to the last man. Colonel Radec was killed in battle against the ISA unit Alpha Squad, who proceeded to enter Visari's throne to finally place him under arrest. Unfortunately, Visari was killed by Sergeant Rico Velasquez out of anger. ISA medics under Narville tried to resuscitate Visari but to no avail. Soon afterwards, the Helghast First Army under Admiral Orlock launched a surprise attack on the ISA, forcing the ISA to make a desperate retreat from Helghan.
The ISA completely failed in their goals to capture Visari and nullifying any chance of ending the war. The ISA invasion force were severely weakened by the detonation of the nuclear weapon in Pyrrhus which killed thousands of their troops, and later devastated by the subsequent Helghast counterattack that destroyed the remainder of the ISA fleet and caused the ISA still on Helghan to become stranded. In the next six months, the ISA would agreed to a ceasefire with the Helghast.
With Scolar Visari dead, the Helghast became now more hellbent than ever on destroying the ISA and UCN for the death of their leader and the devastation of their capital.