From Halopedia, the Halo wiki
Mesra was an independent human colony located in the Qusdar system.
When the Covenant invaded the planet in 2526, it happened to be one of the few human worlds they had attacked so far that was not immediately bombarded from orbit. Because of this, the intelligence analysts of Battle Group X-Ray inferred that the Covenant intended to capture the colony's mining facilities intact. This prompted Admiral Preston Cole to ask that the Militia of Mesra hold off their evacuation to support the UNSC in their denial of these key materials from the Covenant. The Mesranis agreed and vowed to not leave their planet until every last alien there had been killed.
Topography and ecosystem
The planet was known to be mountainous and was at least partially covered in dense, tangled jungles. Within these largely unexplored jungles were numerous forms of exotic fauna, such as arm-length arachnids and mucus-spitting lizards. The planet was most notably home to huge deposits of lanthanide ores, including xenotime. This specific rare-earth phosphate mineral yields ytterbium and erbium, which are essential to the manufacture of ultra-efficient lasers and small-scale fusion reactors. As such, Mesra was very important to the UNSC.
List of appearances