Right! i dont know it either, i hope the next chapter help us to understand better the situation
The tank and the fortress belongs to the mid-eastern combined fleet (the village ish place below the fortress). Mare is at war with with this combined fleet for four years. They were going to trade the original titan with the combined fleet, in exange to support mare conquer elodia. But they failed to kidnap eren and so the war is still continuing. The mare is presently trying to bring down the Slavic fortress which is blocking them from attacking the combined fleet below.
i got it for the most part but just one detail slipped past me, who do the tanks and the fortress belong to? and im assuming the navy they were talking about belongs to marley?