"Threat" is how much of a threat the OF considers the RF at the moment. It will slowly decay unless any of the "modifiers" below occur. It controls how many resources the OF is allocated per 6 hours in-game time.
Threat will not go below a base number, which is the Number of RF bases + Number of RF-occupied towns
+ RF killing OF soldiers
+ RF recently took a village (including peacefully via support)
++ RF is known to have military hardware
++ RF recently took a town/city (not village)
+++ RF recently took a base
- Over time (passive)