The 2017 event Secret Empire was a controversial comic book storyline, mainly due to its depiction of Captain America — or at least, an evil duplicate of Captain America — as a sleeper agent of Marvel's Nazi counterpart organization, Hydra. In this story, the Captain America doppelganger was able to manipulate the superhero community and maneuver himself into a position of power, allowing Hydra to (temporarily) take control of the United States. Unsurprisingly, the heroes who were able to elude Hydra's grasp formed a resistance unit (called, appropriately enough, the Underground) to try and take back the country. Unfortunately, they had no idea that a traitor was hiding in their midst.
In an attempt to keep his daughter Cassie safe from Hydra's clutches, Lang sends her to France to hide. However, Hydra still managed to find Ant-Man's daughter and capture her, giving them the leverage they needed to get Lang on their side. Fearing for Cassie's safety, Lang decides to turn on his allies by giving Hydra vital information about the resistance group's headquarters.
To Lang's credit, he quickly owns up to what he did, lending invaluable assistance to the Winter Soldier by providing him with the Pym Particles he needs to turn the tide against Hydra. The guilt Lang feels due to his actions, however, proves to be too much, so he attempts to leave Earth in "All-New Guardians of the Galaxy" Vol. 1 #12.
