I noticed that reinforcements are based on the largest number of connected spaces of the team.
I surrounded each team to one space. Green and Brown could break one space if they worked together, but brown never attacks that space in-between them.
That's right! Your largest set of connected hexes is your "homeland", identified with a little flag, placed randomly somewhere within that territory. That's where the reinforcements are placed. So, trying to "cut" your opponent's homeland in half in a very important part of the strategy.
And yeah, the AIs don't work together. It's every color for himself!