From Halopedia, the Halo wiki
The Jat-Krula protected boundary or simply Jat-Krula, identified as the Maginot Line in translated Forerunner records, was a Forerunner defensive front that protected the ecumene's core worlds within the Orion complex. It consisted of numerous Line installations, defensive outposts which utilized a variety of measures to prevent enemy ships from entering the line. Millions of these fortifications formed a sphere along hundreds of outlying systems of the Orion complex, intended to cover every possible entry trajectory within both normal space and the higher dimensions of Slipstream space, particularly along the most trafficked and efficient slipspace routes. The Line was originally thought to be an impenetrable defense, since any enemy fleet would be forced to pass through the boundary in order to mount an assault on the systems within, and the entirety of the Line could supposedly be rendered impregnable at a moment's notice.
The Jat-Krula also includes a number of support bases designed to maintain the Line installations. Maintained by archeon-class ancillas and located in systems neighboring those housing the Line installations themselves, these facilities are designed to be indiscernible from natural terrain; for example, Covert Support Base 4276 on Gao was modeled to appear as an intricate system of caverns.
During one of the Forerunners' civil wars over 500,000 years before the Forerunner-Flood war, a legion of Warrior commanders bypassed the line using a novel strategy; by altogether eschewing the crystal-based slipspace manipulation commonly used by Forerunner ships, they flew twenty attack squadrons through the boundary via a non-mediated slipspace trajectory. Though the assault force suffered 50-percent losses, the remaining ships emerged within the sphere and quickly conquered fourteen key systems. This became a notorious example taught to Warrior-Servants at all levels to demonstrate the impossibility of an impregnable defense. However, it did not have a lasting impact on Forerunner strategy in the long term, and the Line continued to be employed as a defense in many conflicts to come.
After the Flood outbreak began to expand out of control during the Forerunner-Flood war, the Forerunner leadership made a decision to leave all systems beyond the Line to fend for themselves, while focusing their efforts on protecting the systems within the perimeter. In the last years of the war, the IsoDidact led Forerunner defense from within the Maginot Sphere, frequently sending transmissions the Librarian, who continued her duties outside the boundary, asking her to retreat behind it to where the Didact's fleets could protect her. Late into the war, the Maginot Line began to fail, with Flood-infested ships breaking through certain sections of the boundary. In the endgame of the conflict, when the IsoDidact was preparing to fire the Halo Array, the Flood, led by the rampant Forerunner AI Mendicant Bias, staged a massive assault on the Line in order to reach Installation 00 and stop the rings' activation. However, the loyal AI Offensive Bias was waiting for Mendicant on the other side with the remnants of the Forerunners' fleet. This led to an enormous naval battle between the two fleets.
The translation "Maginot Line" originates in Forerunner records using translation software which incorporates idioms from the reader's own culture; when Forerunner terms cannot be directly translated the software replaces them with human terms of roughly equivalent meaning. Thus, terms such as "Eden", "fairy tale", and "Maginot Line" serve as the default usage in these records when read by humans. These words appear in brackets in the terminals. Meanwhile, "Jat-Krula" is a transliteration of the original Forerunner term.
The translated name is a reference to France's Maginot Line, a longitudinal line of heavy fortifications meant to repel an anticipated assault from Germany prior to the outbreak of World War II. The line was successful in dissuading a direct attack into France, but Germany was nonetheless able to outmaneuver the Allies and invade France by sending the bulk of their army through the Ardennes Forest in Belgium, which was often considered to be near-impassable by large mechanised armies. The Germans later manned the Maginot Line's battlements as the Allies pushed through Europe, though again the majority of the lines were bypassed. In the popular imagination, the Maginot Line has come to represent military shortsightedness and failed security measures.
List of appearances