Jat-Krula

The Maginot line was a Forerunner defense line that protected the Forerunner inner colonies. It's 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 worlds the Librarian visited.

It was heavily damaged when the Flood made a large attack on it led by the rampant Forerunner AI Mendicant Bias. It was the main line of defense holding the flood back and it is assummed 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.

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 manoeuvred around the fortifications, bypassing them altogether. When the Allies retook France, the Germans manned them, and the lines were once again bypassed instead.