Should a Third World War ever break out, there would be no winner.
Literal
By-chance third world-war [subject-が] occur [something-like-ようなこと-が-あれば], winner [topic-は] cannot-exist-[probably-だろう].
~ようなことがあれば ('should something like X happen') is a more distant, softened conditional — it hedges the possibility by saying 'something resembling X.' Used for remote or undesirable hypotheticals. あり得ない ('cannot exist,' 'is impossible') is the negative potential of ある.