The Cause Of War

H-I-J-K* spells war.

Look,
simply add L-M-N-O:
it's obvious.

Oh, come on,
H-I-J-K-L-M-N-O;
you surely know
that "H" to "O"
is water**.

*Letter sequence nicked from Stephen Rodefer.
**Punchline nicked from an infamous Daily Telegraph crossword clue.