The events leading up to Starfield include a war, the founding of a space detective agency, and another war
Bethesda has published a Starfield timeline stretching from 2050 to the year the game begins, 2330.
Starfield is less than a month away and the drip-feed of information about the space RPG continues: Bethesda has published a timeline of events leading from the fictional colonization of Mars in 2050 to the beginning of the game in 2330.
As you can guess, a lot of that timeline is taken up by humans killing each other. War never changes, and this time it's in space.
The timeline shows that after landing on Mars it would take humankind another century to reach Alpha Centauri, our nearest neighboring star system, in 2156. Humans begin colonizing the heck out of it, so much so that the United Colonies are established just a few years later. In 2188 another alliance of colonies is made, the cowboy-ish Starfield faction called the Freestar Collective, which goes to war with the United Colonies in 2196 (this might be the war with those mechs we saw).
That war lasts a whopping 20 years. Shortly after, the Freestar Rangers is established as an "elite protective and investigative force." Cowboys and detectives? They're the Space Pinkertons, essentially.
Apparently nothing of note happens for another 50 years or so until exploration group Constellation is formed in 2275 by Sebastian Banks, whose name you will probably memorize because I assume you'll be hearing it quite a bit. (Gotta be some sort of holotapes with Banks on 'em at Constellation headquarters.) Another war kicks off as the Freestar Collective violates a treaty in 2308. That's the Colony War we'll probably hear referenced in Starfield plenty, though it only lasted a few years. While the UC and FC are duking it out, Constellation discovers a mysterious artifact and… apparently just sticks it in a drawer. Huh.
Ever wonder why any old citizen in Starfield can have a spaceship with weapons on it, instead of just the military? Well, the United Colonies got their ass handed to them by the Freestar Collective's use of civilian ships and pilots in the last war, so it formed its own civilian navy. That's why you can fly around blasting other ships—official military ships and pilots just can't get the job done by themselves.
As we're nearing the time Starfield begins, we start hearing about some of the people we'll actually meet, like Sarah Morgan, one of the Starfield companions who joins Constellation as a navigator after getting laid off from the UC. Walter Stroud begins financing Constellation, and since he's an incredibly rich dude I'm sure his motives aren't entirely benevolent. Finally in 2326 Barrett (another companion) finds the artifact in Constellation's archives and the group begins searching for more of them. That's where you come in, at the beginning of Starfield in the year 2330, having stumbled on another artifact while mining on Mars.
If you want to skim this timeline for yourself, I can't link to it, exactly: it's on the official website's main page but you have to scroll way down until you see the words "The Story So Far…" You won't have to wait long to continue that story: Starfield launches on September 6, or on September 1 if you have Starfield early access.