Jat-Krula

The Maginot line, otherwise known as the Maginot Sphere, was a Forerunner defense line, or unit that protected the Forerunner inner colonies. Its precise location is unknown, but from records drawn from the Halo 3 Terminals, we can assume that Earth was not inside it and neither were most of the worlds the Librarian, visited in her stays.

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 main and primary line of defense, holding the Flood back and it is assumed it was broken before the first firing of the Halo Array. It was a sphere around a Forerunner controlled area of space.

For a long time it can be assumed that Didact worked in the Maginot line frequently sending transmissions from it to the Librarian asking her to return behind it where he said his Fleets could protect her. Although, it was a defense against the Flood, it cannot be assumed that the Flood conquered all worlds that were outside of it as Earth,- which we know was outside it- was presumably not infected because the Librarian was on it, looking at how beautiful it was before the Array fired.

The Maginot Sphere may have been a physical object,(an energy barrier of some sort) as a Keyship was needed to enter it.

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 tanks maneuvered around the fortifications, bypassing them altogether. Thus proving the fortifications useless, and leading to the fall of France. When the Allies retook France, the Germans manned them, and the lines were once again bypassed. instead.