Infinity War is more than just the third Avengers film, it and its currently untitled follow-up are essentially the culmination of the entire MCU. The long-teased big, purple baddie known as Thanos is scheduled to finally do more than sit in that chair of his and set to work collecting the Infinity Stones for his gauntlet.
In the first released trailer for the film, we're given an idea of exactly how powerful the big guy is, as well as just how successful the gem-hunting is going. One shot in-particular shows Thanos slotting the Space Stone into place. This stone comes from the Tesseract, which Loki stole at the end of Thor Ragnarok, and is an exchange we even see earlier in the trailer.
However, what is of note here is the only other stone we see on the gauntlet; the Power Stone. This purple stone was left in the hands of the Nova Corps on Xandar at the end of the first Guardians Of The Galaxy. Essentially, this can only mean bad things for Xandar.
Seeing as Kevin Feige himself recently teased that Thanos would perform unspeakable acts of horror in the first minutes of the film and that the opening shot of the trailer seems to show a foreign planet being obliterated, Xandar looks to be Thanos' first target.