What is this planet? In the Departure Spartan Ops trailer, they mention the planet New Phoenix that has been attacked by presumably Insurrectionists. Has it ever been mentioned in any previous canon?
It’s a city on Earth. You’ll have to play the campaign to find out what happens there.
Halo 4 Spoilers:
I believe that it is the city that the Didact attacked at the end of Halo 4; the one full of the smouldering ash piles that you see in the epilogue. Frankie said that Spartan Ops takes place 6 months after the campaign, and coincidently it also takes place 6 months after this “New Phoenix Incident”. It also said on that guy’s pad that it was not a Covenant attack either. I think it’s more likely to be the Didact given the circumstances at the time