Allies aren't going to help you if you declare war. It's a mutual-defense alliance - just like your NATO exmple.

However, I don't ever seem to be able to bribe allies into going to war. The option isn't there in the diplo screen. I've never figured out why that is.

From what I can tell the Declare War option is available in the diplomatic interactions with races that are Malevolent.  I've just looked at a save game and the option is available for the Drengin, Yor and Kyrnn despite having only a Neutral relationship with them.  In addition the cost to declare war seems pretty much fixed at around 3,000 credits.  This leads to a system that can be easily exploited i.e, without applying house rules.  In a recent game the Yor were snowballing in strength and I simply asked them to Declare War on everybody else, which softened them up (and other races!) before I declared war.

What the AI should do is calculate a rough opportunity cost of going to war and use that in diplomatic interactions.  Going to war with another powerful race would cost them a lot more a measly 3,000 credits in lost ships etc.  If this was in place I would agree the Declare War option should be made available to Alliance partners.

The reason crossing into another's territory causes no alert or penalty is that, in GC3 as in 2, there IS no such thing as territory other than planets. What you see on the map is in fact your 'influence' and apparently even highly-territorial creatures like Humans don't mind if someone parks a battlefleet a few hexes away.

Now, as for me, I can't get 'territory' out of my head. That zone of influence looks like territory and sometimes acts like territory and so I think it should be territory... but according to Stardock, it is not.

Nope, never had an "ally" respond to a declaration of war on me ever.

It basically seems to do nothing other than act as a way to win by allying everyone. From a strategic viewpoint, no purpose at all.

As far as I can tell, alliance just locks your relations at a high level.

Devs I hope you can work much on alliances in the future

Have you tried 2.0 opt-in? I think in the patch notes this issue was meant to be addressed. I cannot confirm, though.

I agree that in its current form alliances are useless. They are more or less another non-aggression pact.

My suggestions to fix it would be the following:

When the computer are considering war against a civilization they also consider the military strength of the allied. This way it will be possible for military weak civilizations to pursue influence and culture rather than military. But the computer also consider the military strength of its own allies because there are a large possibility they will also join the war.

It would function something like this. When you open up the diplomacy for declaring war a new frame come up with all known civilisations and how they will stand in this war proclamation. On your side, the opponents side or neutral (not getting involved). Allied civilizations have a high probability to stand by the side of their allies, but never guaranteed. The stance of the neutral civilizations will be based on the attacker and defenders power, but of course it will be possible to use diplomatic activities to persuade them to either join your side or stay neutral (if they join the enemy). This means declaring war will take a little longer but then again declaring war should never be routine. My inspiration for this thinking comes from Crusader King 2.

Also in another thread I have stated that I think that the reasons for war should be changed to avoid long wars where nothing happens and create a more dynamic galaxy.