The most common and most likely answer to this crossword clue is the 7 plus 5 letter word MISSION CREEP
I've seen this crossword clue in Financial Times
M
I
S
S
I
O
N
C
R
E
E
P
More clues leading to the result MISSION CREEP
Buddy explains!
The straight part
We know that "War planners try to avoid this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case war planners try to avoid this = mission creep.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.