|“||It's protected by some sort of phasing technology. Our best guess is that destroying all phase generators will drop the protective field around the tower.
- Kirce James, briefing the GDI Commander on the Rome Threshold(src)
During the construction of a Threshold, phase generators are used to protect it through a process called "phasing", making it impervious to all forms of physical damage. Once phasing is complete, the Threshold becomes permanently invulnerable, and the phase generators will no longer be necessary. Each Threshold is protected by multiple phase generators, each of which is enough to render the Threshold invulnerable on its own. Therefore, in order to destroy a Threshold, all the phase generators protecting it must be eliminated first.
When GDI realized that the aliens' attacks on civilian population centers were only diversions, and that their true goal was the construction of the Thresholds, General Jack Granger ordered the GDI Commander to destroy the largest of the alien towers, which was the closest to completion at the time, located in the Italian red zone outside of what used to be Rome. Despite facing resistance from both alien and Nod forces, the Commander managed to destroy the three phase generators protecting the tower, before destroying the tower with an ion cannon strike.
The GDI Commander's victory in Rome set an example for the rest of GDI's forces, and GDI strike teams began destroying Thresholds all over the world. Soon, there were only four towers left standing, and Kane sent the Nod Commander to the Mediterranean red zone to protect the tower closest to completion: Threshold 19, and tasked him with defending the phase generators and eliminating GDI's presence in the area. Meanwhile, Scrin Foreman 371, realizing that the Threshold was its only hope of escape, committed all of its remaining forces to the defense of the Threshold's phase generators.
Despite a relentless GDI assault, the combination of two enemies working against them ultimately proved to be too much for them, and the phasing of Threshold 19 was complete by the time GDI forces fought their way to it, rendering it indestructible. This allowed Foreman 371 to escape back to Ichor Hub in its Mothership, and Kane to prepare his followers for ascension.
- Electronic Arts Los Angeles, Command & Conquer 3: Tiberium Wars. Warp Link, "Threshold Tower Construction".
- Electronic Arts Los Angeles, Command & Conquer 3: Tiberium Wars. Scrin mission 4: "Threshold 19".
- Electronic Arts Los Angeles, Command & Conquer 3: Tiberium Wars. GDI mission 16: "Rome".
- Electronic Arts Los Angeles, Command & Conquer 3: Tiberium Wars. Nod mission 17: "Kane's Tower".
- Electronic Arts Los Angeles, Command & Conquer 3: Tiberium Wars. GDI mission 17: "Ground Zero".