What spells exist for nations to use?

[Oni]

Well by the time that you altered fog cloud to be a battlefield or city sized spell it would be close to the same level as control weather which can create fog in quite a large area for quite a significant amount of time and fast enough to be useful in mass conflict. Beyond that control weather could be used to hinder troop movement with floods, or snowing in mountain passes and the such buying your side more time to prepare. Check out the weather hazards on page 87 of the DMG, while they might not present a lot of danger to a group of adventurers they could be potentially devasting to an army of level 1 warriors.

 

[Xylarthan]

Birthright had lots of great Realm spells and provided for wonderful effects from apparently month-long rituals. The effects would cover one or more provinces or units depending on the spell or the level of the caster.

I'd suggest looking at magic item creation times to work out the time the ritual would take to get the effects desired.