Data relationships for a World Cup Football App

how would I map relationships for world cup football matches.

Currently in my app I have Countries, that's it, all 32 competing
nations are represented, with flags and details working great.

I'd like to introduce two new models (Match and Stadium).

A football match is obviously between two Countries in a Stadium.

How could I set that up best?

Country
  has_many Matches

Match
  has_many Countries
  belongs_to Stadium

Stadium
  has_many Matches

Bit confused as a Match will only ever be between two sides of course.
Do I require habtm? What's best to do here.