From Orberg's LLPSI Cap XLII

[pmda]

Here's from Orberg's LLPSI Cap XLII with my translation.

Cum Lavinio quidem insperata pax erat. Aliud multo propius atque in ipsis prope portis bellum ortum est. Fidenates, cum nimis vicinam urbem prope se convalescere viderent, priusquam tam valida esset quam futura esse videbatur, properant bellum facere.

So
Fidenates, cum nimis vicinam urbem prope se convalescere viderent [The Fidenates, when they saw a neighbouring city grow too strong], priusquam tam valida esset [before it could grow too strong] quam futura esse videbatur, [as they saw it would be in the future] properant bellum facere [hastened to prepare for war]

I'm taking it that 'prope' here is to emphasise that it was a 'near neighbouring' city..

[mwh]

prope se, close to them, on their own doorstep

"before it was as strong as it seemed it would be"
tam ... quam, and videbatur sing. pass.

[pmda]

mwh, Gratias tibi ago.

Paulus