Spoilers ahead (post tagged as such), random thoughts/speculations/reactions of mine a.k.a not my usual analysis-type posts: I knew Kamui & Co. would be back in this arc, I just didn’t know it would be this fast!
The Harusame were divided after the Rakuyou arc. Half went with Utsuro, the other half didn’t. How they reorganized themselves is anyone’s guess; if they reorganized as one force (under the 7th Division’s lead, I’d bet on) or stayed loyal to their respective commanders who remained behind. I’ve always thought that their only reasons for returning to Earth wouldn’t be to protect it because they have no ties to it or any reason worth risking their lives for. Rather, they would return to deal with the other half of the Harusame. The
Three Two Ominous Stars, possibly to return the favour to Joui4 for sparing them in battle (I bet we’ll see them again; old enemies have a habit of popping up unexpectedly or coincidentally). Perhaps they may all uphold their alliance with the Kiheitai, who have now decided to enter the war (which naturally drags in other people on the basis of agreements and treaties).
As for Umibozu, Kamui, and even Abuto I’d wager, the biggest reason of all is Kagura, as expected. So far, we’ve only seen Kamui, but those two probably aren’t far behind (well, Abuto will naturally be with Kamui, and Umibozu is probably going to make an appearance). Another reason is to deal with the Yato tribe that landed there with the Liberation Army, as we’ve just seen (what I gathered, anyway). But they all ought to be quaking under their umbrellas now, because attacking a strong girl with a stronger, very protective father and a stronger, less-estranged older brother…well, you just made the war very personal.
I really hope we’ll see more of the Yato tribe (and female Yato members, please) and gain additional insight into their culture, history, hierarchy, etc. Can’t wait to find out when the Hi Wa Mata Noboru group releases the translated chapter with answers to some questions of mine.