Fiona, Lucina, and Zarya will be aruging that unification will benefit both empires in terms of strengthening each other's military.

Each empire has different, but equally strong military powers that, on merging, would now belong to each other as well. The Sea Clan's naval expertise would be useful for the landlocked Mountain Clan, and the Mountain Clan's overall military prowess would be a boon for the Sea Clan. Of particular note is the Mountain Clan's immediate action during the ambush at the festival, which surely cut down on the number of casualties.

(Zarya in particular is so passionate about this argument that she has tears in her eyes, if that will tug at their heartstrings.)