Probably the framework of friendship is simply defined as a relationship of mutual affection between people.  A stronger bond than acquaintance, but weaker than love.

Simple description aside,  friendship is a favourite study  topic of academia.  The cornerstone behaviours associated with friendship can be very subjective in behaviour, hence the quest to better define the term.

Relationship Driven Behaviour

Typically there is a bond between siblings.  This familial relationship can include friendship, love or even hate relationships but typically there is a positive respect respect and close relationship with your siblings.  (A caring relationship).

Civil wars create a fascinating perspective in the study of relationships.  Friends, family even lovers can be pitted against on another just on the basis of geography.

Do you live in the north and your brother in the south?  Regardless of your relationship, circumstances might drive you to take arms against one another.

The typical behaviour associated with your relationship (e.g, friend) might be transformed to foe overnight.  All for the sake of political expediency.

The point is, typical relationship behaviours can be overcome by circumstance.  Your friend might not act in accordance with your expectations.  It is important to consider the root cause of this disappointment to help you understand what is happening.

Loyalty

A relatively common trait of friendships is loyalty.  You expect support from your friend but circumstances can cause loyalty to waiver (civil war being an extreme case)