What is the meaning of "annihilated in detail"?
attack in detail. Glossary of Civil War Terms
To destroy the enemy piece by piece — by attacking smaller segments one at a time — instead of attacking the entire force all at once.
Using annihilate versus attack is synonymous, as in:
Dublin Review Google Books
The intent is to devote all efforts to a part of the enemy and completely destroy it.
... small bands [to] annihilate in detail immense expeditions sent from time to time against them ...