I'm working on a game where players can setup villages, which can contain defending units. Any of these units (each on their own tiles) can be set to "campaign" which means they are no longer defending but can now be used to attack other villages. And each unit on a tile can have up to a 100 health. So far so good. Oh and it's all asynchronous so even though the server will be aware that your village is being attacked, you won't be until the attack is over.

The issue I'm struggling with, is the following situation.

Let's say a unit on a tile is being attacked by a player from another village. The other player see's your village and is attacking your units. You don't know this is happening though, so you set your unit to campaign and off you go to attack another village, with the unit which itself is actually being attacked by this other player. The other player stops attacking your village and leaves your unit with say a health of 1, which is then saved to the server. You however have this same unit are attacking another village with it, but now you discover that even though it started off with a 100 health, now mysteriously it only has 1...

Solutions? Ideas?

Edit

The simplest solutions are often the best. I referred to Clash of clans below, well after a bit more digging it seems that in CoC you can only attack players that are offline! ha, that almost solves the problem. I say almost because there's still the situation where a players village could be in the process of being attacked when they come back online, still need to address that.

Edit 2

A solution to the "What happens when a player is attacking your village and you come online" issue, could be the attacking player just get's kicked out of the village at that point and just get's whatever they had won up to that point, it's a bit of a fudge but it might work.