@JECE: in the case you describe, you could go for option "B", or you could simply not click anything and write an EoG. Or, when the moment country C attacks D, you could "activate" the alliance at that point by clicking B and write in your EoG that the alliance was automatically activated.
It has nothing to do with International law. These "automated treaties" are no more or less binding than the "treaties" that are signed simply by two players agreeing on a form of cooperation, they are just a different way of *noting* that they did.
Think about chess: whenever you play a chess game, you get a white paper with a yellow carbon paper beneath. On the white paper you note all moves, but if you want, you can flip over that page and annotate the carbon paper. And there's a code for very easily annotating chess moves: a question mark for a bad, two question marks for a very bad move, two exclamation marks for a very good move, etc. etc..
http://en.wikipedia.org/wiki/Chess_annotation_symbols
These annotations simplify the interpretation of a match tremendously, for example, you can see the exact point where an expert player, or the opponent, thought a crucial mistake was made. Then you can dedicate a lot of time studying that particular point in the game. If you look at a Kasparov-Karpov match without some kind of annotation, you simply won't understand why the game was won by one player or the other.