I have been playing MTW for several years, but this is really the first time this has come across my plate.
I had an ally who was invaded by a relatively small army in an adjacent province, who was forced to retreat to his fortress.
I figured "What are friends for" and sent in an Army to lift the seige.
This was considered an act of war by my ally.
Ouch . . . .
I have yet to figure out why alliances were put into the game in the first place, as there seems to be no benefit or penalties involved in making or breaking them.
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