The first culmination point when our heroes get together is in the Avengers. The idea initially began as the Avenger Initiative and was headed by Nick Fury who actually reports to an authority higher than himself- the World Security Council, but we don’t need to talk about them. 

In the present, after tampering with the Tesseract and because the Tesseract houses the Space Stone which can open wormholes, the wormhole is opened by means of a staff which was given to Loki by which Loki enters Earth seeking the Tesseract in order to obtain it for someone much more powerful than himself- Thanos. After being rejected from the throne in Asgard, Loki seeks to enslave the planet which Thor became fond of. After Thanos finds Loki in space he accepts in giving Loki an army of Chitauri to enslave Earth in exchange for Loki obtaining the Tesseract for him. 

Of course, Loki fails and is taken into Thor’s custody as well as the Tesseract and both are taken back to Asgard and Loki is imprisoned for life due to his actions. Next follows Iron Man 3 and despite my opinion on how it does not add to the MCU in any significant way whatsoever, I will mention its comic tie-ins for fan service.