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.