In the 2012 film The Avengers, the alien race that invades Earth is the Chitauri. They serve Loki and attack New York after he uses the Tesseract to open a portal above Stark Tower.
The Chitauri arrive through the portal with a massive fleet and ground forces, overwhelming the city while the newly assembled Avengers fight to protect civilians. Their invasion is coordinated by Loki, who has obtained the Tesseract and is acting with support from a mysterious extraterrestrial power. The film’s climactic battle ends when Iron Man redirects a nuclear missile through the portal and destroys the Chitauri mothership, disabling the forces on Earth.
A common mix-up is calling them Thanos’s army. Thanos is connected to the attack and is revealed in the credits sequence, but the invading species is specifically the Chitauri. The Kree and Skrulls are separate alien civilizations in Marvel stories and are not the race attacking New York in this film.
The Avengers was directed by Joss Whedon and brought together Iron Man, Captain America, Thor, Hulk, Black Widow, and Hawkeye for Marvel Studios’ first major ensemble crossover.