The first thing to understand is when a region asks for reinforcements. The answer to this question is simple. After the first month any region that isn't already being reinforced, doesn't already have 15 Strength and has Desired Strength higher than Current Strength will ask for reinforcements every day. DS is Vigilance + modifiers capped at 15. The modifiers that affect DS are:
- +1 if the region has a Radio Tower. Does not apply to the starting region
- +1 if the region has a Facility or if a Facility is being built there
- +1 if the Protect Region activity is present. Always true after completing Liberation 2
- +1 if the region is the source of an Invasion
- +2 if the region has a Golden Path mission site (Blacksite, Forge, Psi Gate)
- +1 if an adjacent region as a GP site.
- +2 for each adjacent region that is liberated
- +1 for each adjacent region with Vigilance more than 5 points higher than Strength
By default a region will answer the request and send a legion if FS is higher than DS. If more than one region answers only one of them will actually send a legion and it will always be the region with the greatest difference between FS and DS. RNG is only involved to break ties.
A common exception happens when none of the adjacent regions can spare the legion (FS isn't higher than DS), but the region asking for it really needs the extra legion (DS at least 3 points higher than FS). In this case adjacent regions that have very high Strength (9 or more) or those that can't spare the legion but still don't need it that much (DS isn't more than 3 points higher than FS) will answer the request. If multiple regions answer in this step the choice is random.
Other exceptions that force a region to not send reinforcements are:
- A region can't send a legion if doing this reduces FS to zero
- A pair of regions can't send reinforcements to each other at the same time
- A region that is already the origin of an Invasion can't send reinforcements
- Regions with GP sites will only send reinforcements if current Strength is 6 or higher. This always applies to the Psi Gate but only applies to the Blacksite and Forge if the region is already contacted
The other way ADVENT reinforces regions is by using UFOs to create legions instead of moving those that are already there. UFO landing sites are based on RNG, but each region has a weight based on factors that are very similar to those that affect Desired Strength. The initial weight is based on the difference between Vigilance and Strength. The values are:
- 0 if Vigilance is lesser or equal to Strength - 2
- 1 if Vigilance is equal to Strength or Strength - 1
- 2 if Vigilance is equal to Strength + 1
- 3 if Vigilance is equal to Strength +2 or Strength + 3
- 4 if Vigilance is greater or equal to Strength + 4
- +1 if the Protect Region activity is present (Liberation 2 complete)
- +2 if the region is the origin of an Invasion
- +2 if the region has a Facility
- +1 if a facility is being built
- +2 if the region has the Blacksite, +3 if it has the Forge, +4 if it has the Psi Gate
- +1 if the region is adjacent to a region with a GP site