WARNING: This article contains SPOILERS for Avengers: Infinity War
Thanos has decimated the Marvel movie universe in Avengers: Infinity War... and it's all Jane Foster's fault. Technically, Thanos did far worse than just decimate the roster of MCU heroes, reducing all other life throughout the fictional universe by half, including most of the new Avengers. And whether she intended it or not, Jane Foster may be the one to blame.
If "blame" is too harsh a word for the fans of Jane Foster out there (perhaps way out there), then it's at least Natalie Portman's MCU character who allowed Thanos to even hope he could achieve his plan. Not sure of our logic? Well, the fact that Thanos requires all six of the Infinity Stones to launch his final attack is the real key.
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And that simple mission would have been practically impossible... had it not been for Jane Foster's role in Thor: The Dark World.
The Reality Stone: A Refresher
It's been a while since movie fans were first introduced to the Aether - so long, in fact, that many may not have truly grasped its nature as an Infinity Stone in disguise. Back in Thor: The Dark World, it was simply described as a superweapon fueled by the same darkness from which the Dark Elves were spawned (before the universe as we know it exploded into existence, bringing light with it). That was all explained in the film's opening prologue, with Odin narrating his father, Bor's mission to destroy the Dark Elves.
He proved successful, swiping the Aether out from under Malekith at the last second, before the Convergence of all Nine Realms would have let him turn all matter to dark matter across Creation. Malekith fled, perhaps assuming he would try again in the next Convergence (5,000 years later). Meaning the Aether was in the possession of Bor... and too dangerous to keep in the open.
The answer was to find a hiding place for the Chamber that contained the Aether, where it could never be found. And for five millennia, it sat exactly as intended: alone, untouched, and forgotten by the universe.
Jane Finds The Aether, And Brings It Into The Spotlight
...and then Jane came along, and stumbled directly onto it (seriously). Fans may come up with their own explanations or theories to make this section of The Dark World's plot make sense, but even fans will likely agree it's not the most logical sequence of events. The Convergence causes the Nine Realms to overlap, interlace, and teleport people and things from one to another. Fine. That Jane would find one such wormhole while investigating... science readings of some kind? Less believable, but still plausible.
That Jane would, without explanation, happen to stumble onto the one wormhole that would take her - of all possible places throughout the Nine Realms - to the secret, unknown, isolated location of the Aether? Quite a leap, even in a movie universe starring a talking raccoon. That she would press her face into the plot device and be infected with it? Well, let's just say there's a reason why Ragnarok was so rejuvenating for the franchise.
The obvious questions surrounding this plot have never been adequately answered, leaving them as some of the worst Thor movie plot holes to date, where contrived story twists and deus ex machina are concerned.
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The real tragedy is that the minds at Marvel couldn't come up with a reason for Jane Foster to exist other than being fused with the Aether and, as a result, becoming the MacGuffin for most of The Dark World, dragged along by Thor and Loki. But it remains her legacy, after Jane was written out of Thor: Ragnarok.
In the end, her fate was to help Thor care about humans, before bringing the Aether out into the light...dooming the universe to Thanos in the process.
Page 2 of 2: Jane Made Infinity War's Mass Murder Possible
Without Jane, The Reality Stone Would've Never Been Found
Wherever Bor had hidden the Aether turned out to be an ideal hiding place, remaining truly lost since long before Thor, Loki, or Heimdall were born. Until Jane found it. Jane Foster may have washed her hands of her brush with the Aether - by which we mean having an Infinity Stone select her as a host - and who could blame her? But the fact remains, if she hadn't gone searching for "readings" that seem more meaningless than ever now, the Reality Stone may have stayed hidden. Almost certainly would have, since even now we can't say where it was in the universe.
The Reality Stone was ultimately given to The Collector to protect, which Thor believes a stroke of genius, and Star-Lord finds the stupidest possible decision. By the time the Guardians arrive on Knowhere, Thanos has already claimed it from The Collector, so Peter Quill seems to have the stronger case.
And as powerful as the Reality Stone is on its own, had it remained hidden, Thanos never could have wiped out half the universe.
That's due to the fact that the Infinity Stones are far more powerful when joined together. They demonstrate a range of powers on their own, of more or less potency or service to the plot. Without the Reality Stone, Thanos couldn't have gotten the drop on Gamora and the Guardians... couldn't have removed Drax and Mantis from the fight... couldn't have turned Star-Lord's gunfire into bubbles.
As some would argue, it's still possible that Thanos would have turned to other Stones to achieve the same goal (part of the reason why the game-breaking powers of the Stones can make storytelling difficult). But in the end, it takes all six to achieve Thanos's mass culling across the universe. Without the power over reality, it couldn't have been achieved.
Unfortunately, this placement of blame is impossible to state with absolute certainty, since it's not actually clear how Thanos deduced the Stones' locations. Through spying and surveillance, as he did in the comics? Or by simply 'tracking' them (the less interesting option)? If Jane hadn't tripped over the Reality Stone, there would be nobody to even interrogate for the location.
No Jane, no Reality Stone. No Reality Stone, no completed Gauntlet. No Gauntlet... no snap. Natalie Portman may be open to an MCU return, but frankly, she's done enough already.