Fighting alongside your alliance is no longer just a promise. When an alliance member is attacked, your ships at that same location automatically join the defence. And when an alliance member attacks, your armed ships on that spot can join the assault, as long as you are also at war with the target.
Helpers do not start battles on their own. There has to be a real fight between the primary attacker and defender first. A captured planet always goes to the player who launched the attack, whoever did the bombing.
You stay in control. Two new toggles in the alliance settings tab let you decide whether to help defend alliance members and whether to join their attacks. Both are on by default and reset to on whenever you join or leave an alliance. Members you are at war with never assist each other.
Battle reports, the report list and battle news now show everyone who took part on both sides, with ship counts per player and the alliance names of the two main combatants. Every participant receives the report and a notification, not just the two players who started the fight.