The most common and most likely answer to this crossword clue is the 9 letter word MOBILISES
I've seen this crossword clue in Financial Times
M
O
B
I
L
I
S
E
S
More clues leading to the result MOBILISES
Buddy explains!

The straight part
We know that "prepares for war" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prepares for war = mobilises.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MOBILISES mean?
MOBILISES verb- To make something mobile.
- To assemble troops and their equipment in a coordinated fashion so as to be ready for war.
- To become made ready for war.