Memor esto, iam cum signum pugnae dabis, has duas acies spectaculo fore Etruscis, ut pugna fessos confectosque, simul victorem ac victum, aggrediantur. Itaque — si nos di amant — ineamus aliquam viam, qua sine magna clade, sine multo sanguine utriusque populi decerni possit utri utris imperent.
Remember, as you give the signal of the fight, they will attack the two divisions, seen by the Etruscans, tired and worn out by the fight, at the same at the same time victors and vanguished. Accordingly - if the Gods love us - let us take another path, which without great disaster, without much blood of either peoples whether it is possible to see if one could rule the other.
has duas acies spectaculo fore Etruscis – “these two battle lines will be as a spectacle {spectaculo – so-called dative of “purpose”)] to the Etruscans”, “these two battle lines will serve as a spectacle to the Etruscans”.
ut pugna fessos confectosque, simul victorem ac victum, aggrediantur. This is a result clause, introduced by ut. “with the result that [or “so that”] they will attack at the same time both the victor and the vanquished, men worn out and consumed by battle.”
ineamus aliquam viam, qua . . . decerni possit utri utris imperent. – “let us enter some path by which . . . it can be decided which of the two are to rule the others.”
I am always surprised at how the terminologies differ. In all British textbooks (which we in Australia tend to use) this is described as the predicative dative.