Everyone remembers the classic dialogue that went down in the Helicarrier, where Thor declares, "Earth is ready for a higher form of war." Redditor u/SamaritanPrime expanded on their theory, speculating about how Earth has transformed since 2012:
In the first Avengers film, right before Loki’s goons attack the Helicarrier, the Avengers get into an argument that’s exacerbated by the Mind Stone that’s also in the room. During this scene, this exchange happens:
Thor: Earth is ready for a higher form of war.
Captain America: (disgusted) A higher form?
Here’s the thing: Thor was right to say this. Think about what all is on Earth at this very moment:
Steve Rogers is proof positive that the human race can successfully augment itself biologically to fight off other races. Rogers manages to stand up in combat against Loki, Thor, and later Thanos. Tony Stark’s Iron Man suit manages to hold up against the might of Thor himself- and that was just the Mark VI armor. Future upgraded suits could be much hardier than that, and come with Odin only knows what weapons and features.
Wakanda is a nation that has become a technological marvel thanks to its massive supply of vibranium, a metal that is as versatile as it is durable. Wakanda could conceivably arm humanity with sufficient weaponry to defend against a full-scale invasion. Finally, there is an order of sorcerers under the guidance of the Ancient One. This does not factor in the Eternals, a group of beings with mysterious powers.
Thor was right. In less than a century, Earth has gone from a backwater world no one cares about to a rapidly arming world with resources that would make any interstellar power think twice about attacking.