Slitherine

What does having an alliance do for you? Never saw this answer in the manual.

I'm very much a newbie and was wondering the same thing a couple of days ago. The answers were:

a. Guaranteed not to attack you.

b. Quickly get aggressive with your enemies.

c. More diplomatic options.

d. Can make them a vassal.

Unsure how the ethnic group works - whether it is easier to make alliances within the same ethnic group.

Also unsure how it impacts on the Empire score (for happiness in cities).