What is the best definition of an Alliance?

1. Gaining power by conquering and colonizing new lands.

2. Government policies and cultural attitudes centered on war & the military.

3. Two or more countries pledging to defend one another in the event of an attack.

4. The belief that a specific nation, language, and culture is superior to all others.

(i have to choose one of the answers and i’m not quite sure which one it is.)