I'm not positive when or why alliances changed--though I'm not recalling any point in GCIII's development to date in which a faction expected its allies to declare war when it went to war (or when it received a declaration of war). If I recall correctly, something like that was present in GCII.

I'm not sure about Military Alliances.

They should have at least made it so that allies provide you with some sort of support in wars (and not just shared vision) even if they do not enter war themselves. If that being too complex, then they should also enter war automatically as well.

If Military Alliance actually increases your combat power, that's something.  But I'm not sure that's what it does. 

Non-Aggression Pact seems more meaningful, and it's terms are not a subset of alliance.

I suspected it was something like that--that it was the Ship Construction equivalent of the research treaty.

Good point. I'm not sure what the differences are between a non-aggression pact and an alliance (other than tech requirements and relationship strength).

Something nuanced would be nice. For example, before a faction chooses whether to go to war, perhaps it could find out whether its allies will help and, if so, to what extent. When a faction receives a declaration of war, perhaps it could consult with its allies about aid--maybe a meaningful contribution of credits, maybe tech, maybe a gift of ships, maybe a declaration of war against the aggressor, etc. If an ally is unwilling to contribute anything (relative to what it has), this could affect the relationship and may result in a breaking up of the alliance.

Taking this a step further, perhaps a faction that goes to war with an ally might try to get that ally to betray the faction and join the aggressor--the aggressor could bribe the faction, threaten the faction, etc.

That's a bit pie in the sky for GCIII, but as the development team thinks about the franchise going forward, I hope significant thought will be given to making diplomacy and alliances more meaningful.

Following are some (I think) attainable changes to alliances for GCIII:

• Add meaningful differences between non-aggression pacts and alliances, such as some mutual bonuses to research, production, economy, etc., for alliances beyond just an inability to declare war.
• Make alliances require a declaration of war in response to factions that declare war on an ally; otherwise, break the alliance.
• Make alliances part of the equation when a faction determines whether to declare war on a faction (i.e., a faction doesn't just think about the puny "ripe for conquest" faction it wants to conquer, but also factors in the strength of all that little faction's allies--and its own allies).
• Make alliances permanent, but also make it possible to terminate alliances at any point; terminating an alliance would generate a long-term diplomacy malus.

I think those four changes are achievable--in fact, I suspect it would be fairly easy to make those four changes. Doing so would make forming alliances much more meaningful.