[Heartless]

Assuming that your alliance is bigger than any of the attacking alliances...

Primary goal in such a situation should be to limit the growth of your most direct opponent. Reason for this is simple: they have most to gain from hitting you. If you start making it unprofitable for them they are most likely to lose faith into their ability to pick you back. Usually there is a 3rd party that is then catching up on you, but also on your direct opponent, so your direct opponent is losing his position faster. Remember: he is trying to gain something from attacking you, if it looks like he is losing his starting position to someone else without having gained on you then his morale is likely to drop very fast.

How you achieve this goal is then a totally different issue. It helps to crash less value, to not hide production, to not stockpile and to find partners. And to try to use your fleets as efficient as possible, for example by launch-recall-relaunch tactics to gamble on your opponent being less active and thus more likely to waste his fleet slots.