One of the future feature requests by myself (and others) is the ability to change the posture of a side at a scripted time. For example, if you wanted a country to start off as neutral but suddenly declare war at a specified time or in response to a trigger. I came up with a work-around that you see in the attached scenario file. (Perhaps this was already obvious to everyone else, but it wasn't to me.)
In the example scenario there are two sides: side a and side b. I want "a" to turn hostile against "b" at a specific time. So I pick an out-of-the-way location (Patagonia is probably lovely this time of year) and I put down a SAM and radar for side "b". I give side "a" something with eyeballs (SEALs in this case) within visual range of the SAM. Next I put down a single-unit airport in range of the SAM and gave it something to get shot (a target drone, appropriately enough). I then give the target drone a mission to patrol close to the SAM (don't worry, they're neutral! ) that I can set to activate either based upon a time or on another trigger. Finally, I set up an exclusion zone around the SAM that will mark contacts "hostile". Once the mission is triggered, the drone flies into the exclusion zone, gets shot at by the SAM, the observer sees it shooting at the drone, and side "b" is declared hostile.
The downside is that it takes time for the drone to launch and transit to the exclusion zone (~2 min in my example) so you have to compensate for this if you want a time-based trigger. If you activate the mission based upon some trigger condition, then you just have to try to minimize the time by placing the airport close to the SAM. However, you have to be careful not to get it too close such that the SAM site can ID the target as neutral before it gets declared hostile.
I hope this is helpful.