General Antonyms:
* Conflict
* Opposition
* Disagreement
* Feud
* Rivalry
* Separation
* Division
More Specific Antonyms:
* War (if the alliance is formed for mutual defense)
* Betrayal (if the alliance is broken)
* Isolation (if the alliance is dissolved)
* Hostility (if the relationship between the parties is antagonistic)
The best antonym will depend on the specific context of the word "alliance."