I ran across the phrase “annihilated in detail” while listening to Professor Garrett G. Fagan’s instruction regarding the History of Ancient Rome. This comes from a lecture on Marius and Sulla with regards to a particular Roman battle:
They split their forces. As a result when the Germans came on, they were annihilated in detail.
Since the word annihilated already means to utterly destroy, the phrase in detail seems superfluous, not unlike “very unique”. However, a search for the phrase “annihilated in detail” in Google returns enough results that it makes me think that it likely has a particular meaning.
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 …