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No.

Personally, I think it should. Joining an alliance should include an automatic peace treaty with other member nations. Allow the founder to determine the number of weeks for a cease fire if a member leaves the alliance.

I would suggest It should be a very serious commitment to join and alliance, that countries should think longer on who to join and who to let join...thus the alliance should be a treaty of all members with a 4 week cease fire.  With all the planning and investment that's the least that could be done. although, having the founder determine the week count is very fair.

Or you could send a treaty to someone before you invite them to your alliance.

I agree, an alliance should be an auto treaty.

What about this scenario?

A and B are in an alliance, but no treaty.
A wants to destroy C.
A is to the left of B, while C is to the right of B, putting B right between them.
A sends missiles and jets to omgwtfpwn C.

Does B's defenses attack A as they pass over B?

Yes.  So click the "Request Treaty" button.