In 2157 a Citadel Council exploratory vessel is tasked with opening a dormant Relay into unknown regions of the galaxy. What they find is something they never expected. What happens when two civilizations centered around different technologies clash?

The most followed ''Franchise/{{Halo}}''/''Franchise/MassEffect'' crossover on fanfiction.net, [[http://www.fanfiction.net/s/7462757/1/Mass-Effect-Clash-of-Civilizations Mass Effect; Clash Of Civilizations]] by [=Eterna1Soldier=] describes the buildup and ongoing first contact between the humans and alien races of Halo and the alien races of Mass Effect.

Notable for its relatively slow pace, approaching the twentieth chapter before diplomats from either side even ''meet'' each other, it is nonetheless an extremely popular fanfic.

!!This work contains examples of:

* AdaptationalHeroism: Saren. His {{Jerkass}} personality and bigotry against alien species is still present, but without Sovereign's influence he has not turned against the Citadel Council.
* AdventureArchaeologist:
** Liara, who begins the story working on an actual archaeological dig before being recruited to study the new alien species and its artifacts.
** Technically, as someone who is helping with the expedition and analyzing the technology, Tali counts as well.
* AIIsACrapshoot: Averted. Though the Geth remain a dangerous threat from the ''Mass Effect'' series, UNSC AIs are more stable and functional members of their society. They also possess the risk of going 'rampant', but are not inherently unstable.
** Averted with the Huragok, as they are technically AI's, but the worst they do is be totally uninterested in non-technology matters.
* AlternateUniverse: Apart from the crossover nature of the fic, which removes the human Systems Alliance from the ''Mass Effect'' history completely, there are other divergences from the primary ''Mass Effect'' canon. The Batarians became a rogue state 200 years before the start of the series after conflict with the Asari, instead of only recently abandoning the Citadel conventions after conflict with humanity, and the Citadel Council has modified their laws regarding the activation of dormant Mass Relays. As for the Halo side, the orbital installations on Harvest weren't destroyed as thoroughly -- the lift still works.
* BadassCrew: Shepard's Team.
* TheBattlestar: UNSC cruisers and Carriers. The ''Eternal Light of Sol'', a Forerunner ''Fortress''-class warship, takes it beyond [[{{UpToEleven}} Eleven.]]
* BoldlyComing: [[DiscussedTrope Discussed]] by various members of the UNSC and the URS as they converse about various inter-species relationships. Also leads to some initial confusion when [[spoiler: Captain Alenna initiates a meld with Ensign Duran to learn his language. His confused attempts to explain what happened to other UNSC personnel sound like him saying they had just mated]].
* CallARabbitASmeerp: A creature almost indistinguishable from a dog is native to Thessia, known as a "shia", which the Asari likewise keep as pets. [[spoiler: They are not just similar to dogs, they ''are'' dogs (Or at least a close species relative).]]
* CanonWelding: Forerunner Metals are revealed to be the same as the those that make up the Relays and the Citadel.
** [[spoiler: The Reapers were created by the Precursors, and were ordered to leave the Orion Arm of the galaxy alone for two hundred thousand years, which is why the Forerunners and the Ancient Human Empire were allowed to develop to the point that they did.]]
* DysonSphere: as in canon, the world of Trevelyn is a Forerunner-constructed example, currently under the jurisdiction of the UNSC.
* EnergyWeapons: Played straight for the UNSC and Former Covenant. For species that use the Mass Effect though, KineticWeaponsAreJustBetter. Both sides do have points, and the weapons tech on each side is very effective in its own ways.
* FantasticRacism: Saren is condescending and rude to Tali because of her Quarian heritage and dismisses the humans as a brutish and irredeemably violent species before they had even made first contact.
** Though both examples of racism are at bare minimum understandable. The Quarians unleashing the Geth on the galaxy is a point of racial tension that even ''Garrus'' brought up in Canon, and the Migrant Fleet does have legitimate problems that cause trouble for the rest of the galaxy. Humans are somewhat less reasonable, but given that the only evidence of humanity that Saren knew about was a massive battlefield and a burned, scorched garden world (something anathema to the Council Races), it was an understandable first impression.
* EarthShatteringKaboom: When the ''Sol'''s primary weapon was tested on a Brute base, it ''atomized'' the planet and many of the enemy ships in orbit, which was ''not'' what anyone was expecting.
** The crew of the ''Hades Gate'' think that the unknown (Citadel) races did this to one of the moons and put a space station in its place. Turns out that it wasn't a moon, it was just ice and rock that collected around the dormant relay, and activating it caused the coating to shatter.
* AFatherToHisMen: General Fealix.
* FirstContact: The premise of the first arc of the story. Ironically, the contact occurs over Harvest, the same planet where the human-Covenant war began, as the Council forces study the technology and remains of the battles for control of the system.
* FirstContactMath: Notably averted. Each side has their own method of deciphering language, neither of which involve mathematics.
* FirstContactTeam: The crew of the ''Hades Gate'' as well as Liara, Alenna, Saren, Mordin and Felix.
* ForWantOfANail: Though Tali and Liara become friends, without the authority figure of Commander Shepard to smooth over bigotry problems Tali feels resentment, bordering on outright hatred, towards the Citadel Council. [[spoiler: She circumvents Council law to surreptitiously send data to the Migrant Fleet, whereas in the main timeline she had enough faith in her superiors to ask for permission to do so, and would even abide by their refusal if they said 'no'.]]
** Also Saren is a jerkish but ultimately reasonable person who hasn't betrayed the Council due to the First Contact War never happening, so his FreudianExcuse and contact with Reapers likewise never occurred.
* [[GreenSkinnedSpaceBabe Hot Blue Alien Chicks]]: The asari, as [[SarcasmMode subtly]] pointed out by Ensign Duran.
* HumansThroughAlienEyes: The story is often told from the perspective of the Mass Effect races, although not exclusively.
* HumanSubspecies: [[spoiler: It turns out that the Asari do not just look ''like'' humans, but are in fact a species relative. They, and a species native to Thessia known as a "shia", are genetic cousins of humanity and dogs, respectively.]]
* JerkassHasAPoint: Saren is an arrogant, racist bastard, but he takes his assignment as head of security seriously and makes valid points of observation. He is correct that [[spoiler: Tali is illegally transmitting data to the Migrant Fleet, and his armed standoff with ODST troops prevents a bloodbath after Captain Alenna makes a meld at the worst possible time]].
** He also points out a rather significant point- the first contact message that they received only had ''one'' species in it (humans, but they don't know that at the time), despite the war zone they are studying having the wreckage of ''two'' species' ships. From a security standpoint, this is worrying.
** He points out to Tali that most likely, the unknown race with such amazing tech (humans) wouldn't necessarily share it with a race of homeless beggars who couldn't do anything for them in return and whom they barely know.
** Saren correctly points out, after Liara and Tali go on and on about how wonderfully advanced this race of beings is who built all of this, that the only evidence they have of the various species' cultures is a humongous graveyard of warships, as well as a planet that shows evidence of orbital bombardment- a huge no-no to the Council. While the scientist and engineers are acting like [[Franchise/StarTrek starfleet officers,]] gushing about the wonders of the universe, he's pointing out "Hey, we know ''nothing'' about these guys, and what little evidence there is makes them look ''very'' dangerous.
* MentalFusion: The asari use their melding talents to quickly learn languages, and to help those they meld with learn ''their'' language. One of the UNSC's Huragok is able to tap into one of the meld-ee's neural jack, copy the Citadel languages, and provide everyone present with a translator.
* MileLongShip: Every UNSC ship above the weight of a light cruiser and every Covenant capital ship. Averted mostly with Mass Effect ships, as per canon. And like in canon, this is justified- Mass Effect fields on objects need increasingly huge power supplies the larger the object you're trying to have an effect on is. Ships larger than a kilometer (as in dreadnought-class) would require so much eezo that it would be impractical. UNSC and Covenant (and Forerunner) ships, which do not operate under eezo-based tech limits, circumvent this.
* MistakenForProfound:
--> (...)Liara (...) was utterly intrigued with the architecture. (...)Every now and then she would see words written on hanging signs, above doorways, and on walls in some strange alien language. Perhaps they were words of great wisdom or knowledge.
-->''''Restroom Ahead''''
-->''''Dining to the Left (Kids eat free!)''''
-->''''Do not spit over docking ledge. Thank you''''
** Averted when they find the statue of Sif. That actually ''is'' profound.
* TheMole: [[spoiler: Tali sort of; she doesn't have any loyalty to the crew or the Council, only to the Migrant Fleet, so she fed them intel on humanity and the other discoveries the crew makes.]]
* NormalFishInATinyPond: Many of the technologies that the UNSC take for granted, such as artificial gravity, faster than light tech, and crystal computational devices, are utterly amazing to the Citadel Races, largely because they did this without any Element Zero, which was thought to be essential for the first two technologies, at least.
* OOCIsSeriousBusiness: Hev, who is conservative even among its own oblivious-and-not-excitable kind, [[spoiler:after discovering the kinship between Asari and humans.]]
* PoorCommunicationKills- ''barely'' Averted because cooler heads prevailed in time. When Alenna tries melding with one of the humans, they try to separate their colleague from the strange alien who appears to be covering him in weird blue energy. Then Liara uses her biotics to stop them, as interrupting a Meld can have fatal consequences. Upon seeing an alien using psychic powers on their captain, people start reaching for guns, Saren holds the human captain hostage... fortunately, the Meld ends swiftly enough for Kyle and Alenna to explain to both sides what is happening.
* PoweredArmor: The MJOLNIR Mark VIII, made from Forerunner metals that are lighter and stronger and it can be used by non-[=SPARTANs.=]
* ProudWarriorRaceGuy: The Sangheli ambassador, whose purpose seems to be "Let's try to make diplomacy as difficult as possible by being innocently insensitive."
* ReasonableAuthorityFigure: Councilor Tevos has a far more realistic expectation of what possible contact will bring than her two colleagues.
* RubberForeheadAliens: Asari and humans are almost visually identical, with only a differing skin color and the presence or absence of hair. This is a huge surprise and shock to both species, as though they are each familiar with aliens of the same general bipedal shape, they have never encountered such an exact parallel before. [[spoiler:It turns out the Asari are a HumanSubspecies]].
* SchizoTech: The UNSC from the Council's perspective. They have space elevators, massive ships and incredibly advanced computing technology but still use projectile weapons with limited ammunition.
* ShownTheirWork: Ship sizes, technology and weapon effects are taken from ''Mass Effect'' and ''Halo'' wikis and official sources. [=Eterna1Soldier=] has pointed out specific sources when some fans have criticized the sometimes staggering size imbalance between the different universes.
* SpaceElves: The Asari, but this isn't much changed from canon.
* StarfishAlien: The Engineers.
** To a smaller degree, the Hanar, which are noted as looking somewhat similar to the Huragok.
* StealthInSpace: UNSC ''Hades Gate'' can cloak itself, becoming both invisible to light and to other forms of detection. This was an established technology in-universe, as Halo: Reach and Halo: Legends proved.
* ThereIsNoKillLikeOverkill: The UNSC not only send in entire fleets into the neighboring systems of Harvest, but also contact the Sangheili, who in turn send in a Supercarrier, which by itself is powerful enough to conquer a planet on its own. And to cap it off, the UNSC sends in the ''Light of Sol'', a Forerunner Fortress Ship powerful enough to obliterate planets. All of this was a reaction to a small Turian patrol entering Harvest's orbit.
** Justified in-text, as the ''last'' time that humanity met a conglomeration of alien races banded together for mutual cooperation and defense, it triggered a twenty-five year long war of annihilation that reduced mankind to a mere fraction of what it once was. Them showing off the massive resources at their disposal and their military capabilities is their way of saying "We don't want war, but if war comes, we are ready for it, and we can make you sorry you ever so much as ''looked'' funny at us."
* [[Main/ThatDidntHappen This Conversation Never Happened:]] Miles, after discovered that Duran [[Main/WhatDidIDoLastNight probably]] slept with a female Sangheli.
* WhatCouldPossiblyGoWrong:
-->"And besides" Valern said, "even if they prove to be hostile to us, they don't even use element zero. How advanced can they be?"
* WhatDidIDoLastNight: Ensign Duran is very adamant that he did not sleep with a Sangheli that one night....probably.....he thinks. Maybe.
* WhatTheHellHero: Saren gives Tali a vicious, [[JerkassHasAPoint yet accurate]], calling out when [[spoiler: he catches her sneaking info to the Migrant Fleet]], pointing out to her how incredibly stupid of an idea this is.
* WrenchWench: Tali.