From Halopedia, the Halo wiki
Mesra had at least one mountainour region; the Karpos Mountain Range. The mountains were covered in thick and tangled jungle, with a low cloud cover and high levels of fog making air transit largely impossible. The planet was littered with several large cave networks, associated with huge amounts of lanthanide ore deposits; particularly xenotime. As such, the planet was home to a large mining presence, supplying the materials needed for creating small-scale fusion reactors and ultra-efficient lasers to the rest of human space. This made the planet a valued strategic target in the Human-Covenant War.
The planet was home to a varied ecosphere, including the aforementioned jungles. Several forms of fauna were also present on the planet including arm-length arachnids and mucus-spitting lizards.
Millions of years ago, the planet became home to significant lanthanide ore deposits. These ore deposits were notably associated with ancient underground cave systems.
In 2526, the planet became the subject of an invasion by the Covenant. Intelligence analysts within Battle Group X-Ray were quick to notice that the planet was not suffered the same fate of glassing like many other worlds, and surmised that the Covenant were seeking the planet's notable lanthanide ore deposits. The UNSC deployed the 24th Marine Engineering Brigade and Blue Team onto the planet to begin asset-denial operations on the planet's mining infrastructure while evacuating the planet's civilian population. During the evacuation, Admiral Cole requested that the Militia of Mesra stay behind to assist the UNSC force in denying the Covenant access to the mineral reserves; a request accepted by the Militia to an extreme degree - with the Militia forces vowing not to leave the planet until every last invader was dead.
The campaign on Mesra was fought across eight days, with the Marines successful in destroying all of the ore veins and mining infrastructure. By June 8, the 24th were holed up in the Doukala Xenotime Works; the most remote mining operation on the planet and preparing to destroy the mines with multiple nuclear weapons, with Blue Team and the Mesrani militia tasked with holding off the Covenant long enough to enact this plan. Ultimately, the militia were destroyed by the Covenant assault, forcing Blue Team to fall back for extraction by UNSC Night Watch, while the Marines began a premature evacuation, allowing the Covenant victory.
Government and society
Mesra had its own native defensive force, unaffiliated with those of the UNSC Armed Forces or Colonial Military Administration. Instead, the planet had its own militia force. The militia had numerous brigade-level organisations, indicating a force of multiple tens of thousands of serving personnel. During the Covenant invasion of the planet, the militia allied itself with the UNSC.
Mesra had a large mining industry, digging up and refining the lanthanide ores that were common in the planet's crust.
The citizens of Mesra went by the demonym of Mesrani, though often referred to themselves as the "Children of Mesra". The Mesranis had a staunch stance against the use of stim-packs and possibly other drugs; claiming that their original intent for settling the planet was to get away from the UEG's more lax stance on the issue. The Mesranis additionally placed a great value on keeping their word once given; vowing to not leave their planet until every single invading alien was killed on it. This oath was considered extreme even by the SPARTANs of Blue Team, and the Mesranis so focused on this that they eventually led to their own deaths and mission failure over viable alternatives for evacuation. 
The Mesranis were considered by John-117 to have an egalitarian leadership, preferring to pursue goals that would satisfy everyone under a commander's leadership over what was considered the most sensible or viable option. During the campaign against the Covenant, the Mesranis placed a great deal of emphasis on vengeance against the Covenant over merely fulfilling their objectives. The Mesranis also looked down on the strict adherence to rank in the UNSC military, instead preferring a more informal leadership style in which honorifics such as rank or "Sir"/"Ma'am" were not needed.
List of appearances