(ii.) _Translation._--The form of the sentence shows that it is _reported speech_, and not the actual words of the speaker +Centenius+, who is still the principal subject, and +dixit+, _understood_, the principal verb, and +se peritum ... usurum+ the object of +dixit+. You should now be able to translate without any difficulty, and the logical common-sense rules for the conversion of Or. Recta into Or. Obliqua explain the mood of the verb +capti forent+ in the subordinate clause introduced by +quibus+.
Literally: _Centenius said that he, experienced in both the enemy and the districts, would soon make it worth (their) while: and that he would use against their inventor those arts by which up to that time both our leaders and our armies had been overcome._ Notice that the long relative clause +quibus artibus ... forent+ is in Latin placed before the antecedent +iis+.
You will readily see that this must be improved in several points. Thus:--
(a) +Use Oratio Recta+--more graphic and better suited to our idiom.
(b) _arts_. Change this to some more suitable military term--e.g. _tactics_.
_He was well acquainted (he said) both with the enemy and the country, and would shortly make it worth their while, and would employ against their originator those very tactics by which both our leaders and our armies had up to that time been baffled._
+V.+ +Id non promissum magis stolide, quam stolide creditum: tamquam eaedem militares et imperatoriae artes essent!+
(ii.) _Translation._--The finite copula +est+ is, as often, omitted; the two principal verbs are +promissum (est)+ and +creditum (est)+ linked by the comparative particles +magis--quam+, and the subject is +id+; +tamquam--essent!+ is a subordinate clause modifying the two principal verbs, and expressing contemptuous wonder.
Cf. ‘+tamquam clausa sit Asia, sic nihil perfertur ad nos+.’
Literally: _That was promised not more foolishly than it was foolishly believed, just as if the arts of a soldier and of a general were the same._
Here you can make several improvements; avoid the repetition of _foolishly_, and use a better term than _arts_, and perhaps break up the sentence into two short ones. Thus:--
_The folly of the promise was not greater than that of the credit it received. Just as though the qualities of a soldier and of a general were the same!_