Does it makes sense that the defending fleet of a system is forced into combat when any of the planets is attacked?

It would be logical for the defending fleet to be able to "chose" its engagements and letting low-priority planets be captures staying back for the one important planet in the system.

Else, if the defender has two planets in the system: ( one high-prio industrial powerhouse with planetary defenses and star bases and a low prio moon with no buildings and 1 pop), the attacker can attack the low prio planet with no defenses and for the defending fleet from
1. retreating from the whole system leaving the high-prio planet undefended + taking losses
2. Taking losses defending the low-prio planet without defenses of its own

Though it would make sense that the defending fleet has a "ignore invasion" option, I am not sure it would make sense game-wise:
The AI seems to already use the above mechanic and invading my low-prio planets which is absolutely fine if this is an intended mechanic, but if not, "ignore" would be a logical choice but AI would need to be adjusted to this consideration

Edit: Seems that auto retreat places the fleet back to the same star system... So this is kind the "ignore" option then?
Also: Can evacuate planet during invasion which is cool if there is a defending fleet to cover their escape but shouldn't they be caught by the attacker?

There is a basic principle to how movement and combat works in the game: if you are attacked, you must either defend or flee the system. If a fleet could simply ignore attacks, then it could stay in a system indefinitely regardless of enemy attempts to attack it.

This does lead to the slightly strange case in which the player has more than one colony in a system, and the enemy attacks the more weakly defended colony. However, if you want to give up the weaker planet to the enemy, you can just choose to defend and then retreat from combat; if you still hold another colony in the system, your fleet will stay in the system. We could add a conditional "ignore" button that appears only when the defender has more than one colony in the system, but that would be a little bit irregular, and it's a non-trivial amount of work to implement just to save one extra button click.

When an enemy fleet is in the system, the system is blockaded and transports can't escape. If you want to evacuate a colony, the time to do it is before the enemy fleet is attacking.

Arioch wrote:There is a basic principle to how movement and combat works in the game: if you are attacked, you must either defend or flee the system. If a fleet could simply ignore attacks, then it could stay in a system indefinitely regardless of enemy attempts to attack it.

This does lead to the slightly strange case in which the player has more than one colony in a system, and the enemy attacks the more weakly defended colony. However, if you want to give up the weaker planet to the enemy, you can just choose to defend and then retreat from combat; if you still hold another colony in the system, your fleet will stay in the system. We could add a conditional "ignore" button that appears only when the defender has more than one colony in the system, but that would be a little bit irregular, and it's a non-trivial amount of work to implement just to save one extra button click.

When an enemy fleet is in the system, the system is blockaded and transports can't escape. If you want to evacuate a colony, the time to do it is before the enemy fleet is attacking.

I see. That explains all the enemy fleets that have fled and stayed in the system nonetheless - thought originally to be a bug

Regarding transports evacuating: I just had a game where I could evacuate my pop before attending to the defend action against an invasion on the same planet. Not a big issue though, haven't seen this turning out to be an issue (AI does not seem to do it)