...se eorum opera usurum...

[pmda]

Priusquam inde digrederentur, Tullus Mettio imperat uti iuventutem in armis habeat: 'se eorum opera usurum [usurum esse], si bellum cum Veientibus foret [foret = esset].' Ita exercitus inde domum abducti sunt.

Before they departed, Tullus commanded Mettius to keep his youth under arms: so as to be of use in defending their freedom, should war occur with the Veienii. in this way the army was led home from there.

[Qimmik]

se eorum opera usurum . . . -- "[Tullius said] he would use their work/efforts . . . " Not sure where you get "defending their freedom"

ita exercitus inde domum abducti sunt -- "Thus, the armies were led home from there." Note the plural.

[pmda]

'se eorum opera usurum . . . -- "[Tullius said] he would use their work/efforts . . . " Not sure where you get "defending their freedom"

ita exercitus inde domum abducti sunt -- "Thus, the armies were led home from there." Note the plural.'

I spent a while figuring out that Opera wasn't plural but was ablative singular, - thinking maybe 'se' was ablative in some way following usurum.... then another few minutes figuring out (or not) who 'se' was referring to. ..and then I looked up a definition of Opera in L&S and one of the definitions said 'freedom'....

....a catalogue of errors.

Plural exercitus noted.

as ever thanks for your guidance....

[Qimmik]

"Service" might be a better rendering of opera here.