。
All's fair in love and war.
Literal
Romantic-love [and-と] war [in-では] means [object-を] do-not-choose.
A direct Japanese rendering of the English proverb. 手段を選ばない literally 'does not choose means' = 'is not picky about methods,' i.e. anything goes. The では here marks the domain ('in love and war, as for them'). Note the plain negative 選ばない has a general, gnomic reading, typical in proverbs.