Iron Man 2

These, plus The Incredible Hulk, all occur within roughly a week of each other — fan-nicknamed "Fury's Big Week."

Iron Man 2: The arc reactor is poisoning Stark. Ivan Vanko, son of a Stark Industries defector, attacks him; rival contractor Justin Hammer backs Vanko. Stark synthesises a new element from his father's old research, cures himself, and — crucially — is assessed by S.H.I.E.L.D. agent Natasha Romanoff and judged unsuitable for the Avengers. He's recommended as a consultant only.

His friend James "Rhodey" Rhodes takes a suit and becomes War Machine.

Loki does not die. He falls into the hands of Thanos, who gives him an army and the Mind Stone.