I really like the idea of alliances being made up of smaller teams because that seems to me like a more realistic depiction of large-scale conflict, historically speaking at least. Big tent alliances can lead to an interesting variety of interests working towards some common goal (like say the American republic, the Soviet Union, and the British Empire), and to me this possibility of throwing together groups that might not seem like natural allies at first glance improves my reception of Sarah's idea of not announcing the composition of the two alliances closer to the start of the actual event.