I would like to propose a proper alliance system where a corp could send a (alliance/war) treaty to another corp. Example:
Corp 1 offers Corp 2 an alliance
Corp 2 accepts
Now Corp 1 is attacked by Corp 3
If Corp 2 has no alliance treaty with Corp 3, all of Corp 2's ships in the same orbit will take part in the battle.
You could make it deeper by making different kinds of 'alliances', for example:
Alliance Treaty 1 will have Corp 2 taking part in battles if Corp 1 is the one being attacked, but if Corp 1 is the attacker, Corp 2 will not take part
Alliance Treaty 2 is the vice versa of Alliance Treaty 1
Alliance Treaty 3 is a combination
Alliance Treaty 4 will only help if the winning chances are higher than 50% or something like that.