Noob question about AI and diplomacy

[sum1lost]

Lazy_Perfectionist said:
If I've got conveniently placed independent scout recruiting points, I churn out scouts nearly nonstop until I've saturated nearby nations. I even find it helpful to place scouts in the lands of trustworthy allies. If they're facing a deep, hostile invasion, it will let me know precisely what I'll be facing if I launch a relief effort, or aim assisting assassinations.
Or, if the invading force moves on without leaving any province defense or troops behind (it happens), I get to mess with their lines of retreat.

This is how I did a good deal of damage to Man, including destroying his retreating knights and prophet-hero-commander.