The expansionism was a funny one. Supposedly space in the Milky Way galaxy is in short supply between the Sangheili and Humans and that Human "expansionism" (Whatever that actually means; we have seen absolutely nothing so far that defines what that is in comparison to other races' supposedly "acceptable" expansions. It's utterly meaningless drivel that we are just expected to accept all of a sudden.) is a serious issue for the Sangheili. It is also made out that all of a sudden, the two empires that had never even known of each others existence until 27 years ago are now suddenly rubbing uncomfortably against each other. (The Sangheili have been space faring for almost 4000 years and never once did Humanity detect their radio emissions or anything. They can't be that close surely.) All in all, just another shoddy excuse to get the outdated conflict ball rolling again.