A basic lore rundown of what I've heard from both past and present events for anyone lost on the lore. Feel free to add points and further details / theories in the comments.
Thargoids came before Guardians.
Thargoids are environmentalist, and decide not to expand throughout the entire galaxy.
Guardians pop up, and begin making crazy technology while colonizing and possibly terraforming other planets.
Guardians (or some other race) come to Sol and leave behind technology on Mars.
Guardians develop a very powerful AI, which is capable of independent thought and replication (essentially a grey goo).
Thargoids don't like that, and tell the Guardians to chill, but the Guardians responds with the middle finger, and the Guardian-Thargoid war begins.
AI kills off Guardians in a Skynet maneuver, and Thargoids are trying to stop it.
AI is suppressed, and Thargoids will attempt to cease any further Guardian activity.
Thargoids reverse-engineer Guardian technology, like the shutdown field (used against the AI), and Witchspace travel via artificial wormholes.
Humanity pops up.
Nuclear war occurs, and corporations overtake the government.
Humanity begins space exploration, and construct Generation Ships in case something bad happens on Earth again (the 70,000 constructed point was actually a typo, but has become a widely distributed fact over the span of a millennium in Elite lore).
Humanity discover the leftover alien technology on Mars, and use it to create hyperdrives.
A bunch of political and corporate squabbles, where technology is made and lost (other Elite games).
Humanity encounters first Thargoid, and encounters were mixed in response.
Thargoid envoy is sent to New Africa for peace negotiations, but is killed by humans.
First Human-Thargoid war begins.
Azimuth captures Thargoid test subjects and create the mycoid virus.
CMDR John Jameson (person you play as in previous Elite games) is sent by Salvation (the Witch) to deliver the mycoid payload to a Thargoid Hive Ship, effectively destroying it.
Thargoid Hive Ships are either destroyed or grounded around human-occupied space, creating Thargoid surface sites.
First Human-Thargoid war ends.
The FSD is developed (or rediscovered, possibly reverse-engineered from Thargoids), and humanity begins rapid expansion (the Bubble).
Humanity unknowingly trespass on Thargoid territory, and begins harvesting Thargoid barnacle sites without asking.
Thargoids don't like that, and tell humanity to chill, but humanity responds with the middle finger, and the second Human-Thargoid war begins.
Humanity discover Guardian weaponry, which is used to counter Thargoids.
Salvation experiments with captured Thargoids, and creates a Human-Thargoid hybrid labeled as D-2 (Seo Jin-Ae), which may have aggravated the Thargoids into returning to humanity.
Thargoids mainly attack human systems far outside of the Bubble, to try to keep humanity suppressed.
Salvation uses stolen Guardian technology to lure Thargoids into human inhabited space, and test-fires the Proteus Wave, which is the original Guardian technology that they used to try to stop their own AI, and what Thargoids reverse-engineered.
Salvation hooks up Proteus Wave to grounded Thargoid Hive Ship, and fires to destroy all Thargoids within the area.
Hive Ship adapts and repurposes Proteus Wave to work against human technology, and sends out a distress call, which is answered.
Thargoid surface sites begin accepting and repurposes inserted Guardian relics.
UIA (Stargoids)s begin mobilizing towards Sol, using reverse-engineered Proteus Wave to lay waste to any humans in their path.
D-2 is attempting to communicate with Thargoids to stop the conflict.
Source: https://www.reddit.com/r/EliteDangerous/comments/yzq4sw/basic_lore_rundown_with_some_theorizing/
