"a player can never break your alliance"
We had some questions about this as well...but they were more about what happens when you have an alliance with a player and he moves the meeple out of the shared space. Can he? doesn't that break the alliance? or are the alliances not based on sharing a space? (this is also gets to a player moving a piece out of a shared territory, then moving it back in to get the halfling bonus (level 2 I think)...we ruled that you couldn't do it because it was silly).
So the actual question is what constitutes breaking an Alliance? Is it only the betrayal in combat step?
After the halflings reach level 4, if they have an alliance with a player that player cannot break the alliance. This includes moving one of their meeples out of their last shared region. (They can move meeples out of other regions, provided there is at least one shared space)
Mechanically, hitting level 4 locks in your currently alliances. Then, when you hit level 5, you get the bonus VPs for them. Alliances with halflings in the game are very tricky... you need to be very wary of when they are going to hit level 4. You could get locked into something you don't want to be a party of.