The heroes already didn’t trust each other because of Civil War, so a complication of this scale wasn’t helping matters.
The Skrull were first introduced in 1962 and have primarily been enemies of the Fantastic Four, though they have troubled other heroes as well.
Perhaps their most prominent story is Secret Invasion, in which it’s revealed that the Skrull have replaced several significant figures on Earth in a long-term plan to take over the planet.
It was a vast comics event that crossed over dozens of different series, and it would make for a fascinating movie arc.