It’s a little weird that breaking a formal alliance involves a game wide announcement, but forming one does not.
Suggestion: Make two types of alliances available: Secret (for $250) and Public (for the standard $150). Public Alliances involve a game generated announcement of the two players entering into a formal alliance. Paying the extra for a secret alliance means no such announcement is made (as it is now).
In both cases, when an alliance is broken, there is a game generated announcement.