The "Kree/Skrull War" is considered to be a highlight of Avengers comics from the 1970s and one of the great cosmic tales of the Marvel universe. As its name suggests, it dealt with the massive intergalactic war between two alien species – the Kree and the Skrulls – as it encroached upon Earth territory and threatened to engulf all of humanity. Just another day in the Marvel universe, am I right?
For a story that had the word "war" in the title, there was very little in the way of actual militaristic combat going on. Still, the tale more than made up for that with copious amounts of cosmic weirdness thanks to appearances by Captain Marvel – not to be confused with DC's Captain Marvel or Marvel's Marvelman – Ronan the Accuser, the Super-Skrull, Annihilus, the Inhumans, and those Skrulls that the Fantastic Four had hypnotized into thinking they were cows, A.K.A. the Cow-Skrulls.
Alas, the "war" was effectively ended with the help of a good, old deus ex machina – Avengers sidekick Rick Jones was magically given the power to just stop both warring sides from fighting with the snap of his fingers – but the events of the storyline are worth reading up on, as it's quite possible they'll influence the subject matter of future Avengers films. Maybe in Avengers 4?