When war begins Hell opens.

[Quando la guerra Comincia, s’apre l’inferno.]

James Howell (c. 1594–1666) Welsh historian, writer, aphorist

Paroimiographia [Παροιμιογραφία]: Proverbs, or, Old Sayed Sawes & Adages, “Proverbs in Italian” (1659) [compiler]
    (Source)

Also recorded by Herbert (1651).