Article issue

At last the war ended, but the transition from … war to peace was painful for both sides.

(a) the
(b) -

My friend has told me that (a) is correct,
but my option is zero article, since there is a “from war to peace” expression.

Who is right?

(b) is more natural, with “war” used in an uncountable sense.

(a) is not impossible.