Which movies serve as prequels for Infinity War? Not the Failtastic Four or X-Men as those are Fox properties, not Disney.
In sequence, by release date - with some eliminated due to no connection (via the plot or infinity stone) to Infinity War or due to being a really lousy movie with a tenuous connection Infinity War. Any I'm missing that connect to Infinity War?
- Captain America: The First Avenger (infinity stone)
- The Avengers (I'd consider all the Avengers films prequels to Infinity War, which is an Avengers film in itself)
- Thor: The Dark World (Loki, infinity stone)
- Captain America: The Winter Soldier (great movie, in addition to close tie-in to Civil War; also infinity stone in credits)
- Guardians of the Galaxy (Thanos, infinity stone)
- Avengers: Age of Ultron (Avengers)
- Captain America: Civil War (Avengers)
- Doctor Strange (infinity stone)
- Guardians of the Galaxy Vol. 2 (Thanos)
- Thor: Ragnarok (dunno, but assume Loki and infinity stone tie-in; also Hulk returns after disappearing in Age of Ultron, and just before his appearance in Infinity War)
- Black Panther (Wakanda, which seems to be location of a major battle in Infinity War; also likely location of the last stone, the Soul Stone - yes, it's only natural that the Soul Stone would be in Africa because everybody knows whitey ain't got no soul - you can't make this stuff up . . . except Marvel (likely) did)
The full Disney - Marvel Cinematic Universe list:
Iron Man (2008)
The Incredible Hulk (2008)
Iron Man 2 (2010)
Thor (2011)
Captain America: The First Avenger (2011)
Marvel's The Avengers (2012)
Iron Man 3 (2013)
Thor: The Dark World (2013)
Captain America: The Winter Soldier (2014)
Guardians of the Galaxy (2014)
Avengers: Age of Ultron (2015)
Ant-Man (2015)
Captain America: Civil War (2016)
Doctor Strange (2016)
Guardians of the Galaxy Vol. 2 (2017)
Spider-Man: Homecoming (2017)
Thor: Ragnarok (2017)
Black Panther (2018)