Base Delta Zero (Δ) was the naval code for a type of orbital bombardment capable of completely destroying the surface of a planetary target. It was used by both the Galactic Republic and its successor state, the Galactic Empire, although the former only used it as a last resort against enemies entrenched deep underground, while the latter deliberately used it as a tool of planetary-scale extermination.
Base Delta Zero (Δ) was an extreme form of orbital bombardment capable of melting a planet's upper crust to molten slag, which in practice could amount to completely destroying said planet's population.
The Base Delta Zero initiative was originally introduced by the Galactic Republic, as a last resort against enemy armies entrenched deep underground. After the Republic was replaced by the Galactic Empire at the end of the devastating Clone Wars, the code remained in use. However, the Empire publicly claimed that Base Delta Zero was only involved in planetary liberations, while in actuality the tactic was used for mass exterminations. Prior to Clone Force 99's in around 18 BBY, the Empire conducted a Base Delta Zero on the Imperial medical laboratory on the planet Setron after the Advanced Science Division lost control of one of its experiments, the slither vines. The bombardment destroyed the Imperial facility but left a trio of clone cadets who managed to escape the facility prior to its destruction.
In her journal, the Mandalorian artist and freedom fighter Sabine Wren wrote about how the Empire had lied about the true meaning of the code. On one occasion on Lothal, Wren managed to access the data contained within the AJ^6 cyborg construct worn by Tseebo, an employee of the Imperial Information Office. There, she saw information about a five-year plan for the Empire's development in the Outer Rim Territories. When she reached a screen featuring the characters Δ, Wren was barred further access.
Within the Empire, Base Delta Zero was also part of everyday slang. When Zare Leonis, cadet at the Academy for Young Imperials, began fighting with Nazhros Oleg after being provoked, Lieutenant Currahee warned their unit, Aurek, that she would be "going Base Delta Zero" on them if there were any further screw-ups.
The Base Delta Zero naval code was first introduced in Star Wars Legends in 1989 in the Imperial Sourcebook for West End Games's Star Wars: The Roleplaying Game. During early development of the Star Wars Rebels animated series, the idea of Lothal being faced with a Base Delta Zero event in the end-point of a storyline was proposed, but never utilized. The phrase became an easter egg that appeared throughout Season One.
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