The season opener of Agents of S.H.I.E.L.D.‘s final run teased a major connection to a cornerstone of the MCU, with HYDRA shown to get hold of a bunch of vials filled with a mysterious green fluid. Fans wondered whether these could be an early form of the super-soldier serum that would go on to create Captain America. In the second episode of season 7, this theory was confirmed, with new information revealed about how the serum came about.
The team have traveled back to 1930s New York in a bid to stop the alien Chronicoms from altering Earth’s history and preventing S.H.I.E.L.D. from forming. Their first gambit is to prevent the creation of the super-soldier serum because without that, there’s no Red Skull or Cap and the changes to the timeline would be astronomical. Without the rise of both the S.S.R. and HYDRA to infiltrate it, S.H.I.E.L.D. would never be founded.
As Jemma Simmons notes in episode 2, Dr. Abraham Erskine first developed the formula in his native Germany in 1930. It’s confirmed that HYDRA was watching him from the start of the process, and those vials contained a crucial ingredient to the serum that they’re planning to ship to Germany from the U.S., which the Chronicoms aimed to intercept. Given that they have to uphold the timeline, though, the team had to make sure HYDRA got away with it.
This info builds on what was previously established in The First Avenger tie-in comics. These detailed that Erskine realized what the Nazis really wanted his serum for in 1935 and attempted to flee his homeland. However, Johann Schmidt locked his family up in a concentration camp (in which they tragically died) until he perfected the formula. Thankfully, Peggy Carter led a mission to free Erskine and he defected to the U.S., developing the serum for the S.S.R. instead.
Remember, there’s an Agent Carter crossover coming up this season, so it seems like there could be more Captain America connections on the way before Agents of S.H.I.E.L.D. reaches its thrilling conclusion.