If someone tries to achieve something by fair means or foul, they use every means possible in order to achieve it, and they do not care if their behaviour is dishonest or unfair.
They will only be satisfied if they regain control–by fair means or foul.
See full dictionary entry for foul
by fair means or foul
by any possible method, even if it is dishonest or unfair
She never gave up trying to recover her property, by fair means or foul.