Why my allie didint join the war?

[Luky]

Not even he didnt join but he has still open bordersa with enemy so the enemy sent fleets through his empire and attack me "from behind" and I am ♥♥♥♥♥♥

[Elementus]

your ally has no need to go to war against that empire

[Elitewrecker PT]

You gave us barely any information.

[Ihateeverybody]

Friends...how many of us have them....Friends....Ones we can rely on.... Friends......before we go any further lets be Friends....(Its a song....I think from mid to late 80s, can't recall).

Sorry I am old.

Trust no one and no thing. Relying on others to act in your interest is foolish.

[Malaficus Shaikan]

From my understanding of how the ai works:
There enemy power level(as seen in the diplomacy screen) vs there own power level.
Distance.
Risk of loss.

The ai problem doesnt take into considering your combinded might only its own.
It will avoid wars it cant win and wont go to war unless they can accauly inflice damage without going to far from home/risking too many ships.

For example:
Destroy a fanatic purifier fleet and watch the serounding ai slaughter then remains.
Attack an nation that is stronger then them and they cower in fear.
One of the ways i get my allies to "help" is to lure and destroy the enemy's fleet.
When the ai programing see's that your enemy is weak it will quickly move it force's to capture whatever it can.

As a general rule:
Allies and vassals are for capturing minnor systems you dont want to send your fleet towards.

[Mistfox]

Did you invite him to join the war? If you left him out, it's not his fault he's sitting there doing nothing. Not to mention what type of "ally" is he? Non-Aggression? He's not obligated to join even if invited. Defensive Pact? If you're attacking, he's also not obligated to join. Guarantee Independence to you? Same thing. Federation? Yes, now he's obliged to join.

So what kind of "Ally" is he?

[_ALuX_]

If you have a defensive pact with an ally before you declare war, you need to invite them via the war declaration screen. They won't join on their own accord.

The other options you have are to either wait until your ally declares war themselves OR if you or or ally is attacked first (provided you have a defensive pact in play of course).