Jat-Krula

The Maginot Line, otherwise known as the Maginot Sphere, was a Forerunner defensive line that protected the Forerunner inner colonies. It divided what the Forerunners were able to protect from what they had to leave to the Flood. Its precise location is unknown; however, Earth was most likely not inside it and neither were most of the worlds the Librarian visited in her duties.

It was heavily damaged when the Flood made a massive attack on it, led and commanded by the rampant Forerunner AI Mendicant Bias. It was the primary line of defense holding the Flood back. It is assumed that it was broken before the first firing of the Halo Array. Although it was a defense against the Flood, it cannot be assumed that the Flood conquered all worlds that were outside of the Sphere, as Earth (which was suspected to lie outside of the sphere) was presumably not infected, because the Librarian was on it, marveling over its beauty before the Array fired.

It can be assumed that for a long time, the Didact worked in the Maginot sphere, frequently sending transmissions from it to the Librarian asking her to retreat behind it to where the Didact said their Fleets could protect the Librarian.

The Maginot Sphere may have been a physical object, (an energy barrier or a defensive line of some sort) as a Keyship was needed to enter it. It may also be a hypothetical line, not a physical object, used to mark what could and could not be saved, and the fleets guarding it would not permit anyone or anything to pass unless it was in a Keyship.

Naming
"Maginot Line" was not the actual name used by the Forerunners to refer to the defensive perimeter. The Forerunner records describing the line were discovered using translation software so advanced that it incorporated idioms from the reader's own culture, thus where objects with no actual translation were referred to, the software replaced them with Human terms of more or less equivalent meaning. Thus, terms such as "Eden", "fairy tale" and "Maginot Line" became the default usage in Forerunner terminals and transmissions when read by Humans. These words appear in brackets in the Terminals.

Trivia

 * The Maginot Line was a World War II French line of fortifications, meant to repel an anticipated assault from Germany. Though it dissuaded a direct attack, German tank units maneuvered around the fortifications, bypassing most of them altogether, making the fortifications mostly useless and leading to the fall of France. When the Allies retook France, the Germans manned them, but again the majority of the lines were bypassed.
 * Line Installation 1-4 seen in the Halo: Blood Line comic series may well be part of the Maginot Line. This is supported by the Installation's name, as well as its function which includes preventing ships passing through a certain point, using a weapon that can fire into Slipspace, as well as research for immunity to the Flood. In addition, Frank O'Connor commented on the name that "fans of the terminals will probably figure it out", implying that the reference to a "Line" is linked to the story told in the Terminals, where the Maginot Line was first mentioned.