That was often the exception rather than the rule...

Historically:

- Achmeneid persia: 3 major battles to cripple decisively their army

- Seleucid Empire : A bit more comples, but Raphia and Magnesia were pretty bad hits and they never recovered from those

- Carthage: it was long but after the first punic war (in which after two major naval losses they capitulated) it took less than 10 losses (baecula, dertosa, ilipa, great plain, metaurus and zama, might be a couple more) to reduce them to little more than a city state

- Ptolemies: a long survivor but for the longest part of their history they were little more than a client state of Rome

The point is: even if you have a huge empire professional soldiers aren't going to be very abundant. You can recruit huge armies composed of levies but everybody knows what happens when levies faces veterans...

Rome was the exception mostly because they found a way to turn their levies into an effective fighting machine and had a more efficent training system.
This is especially true in our game timeframe, where Rome won more than a couple wars through attrition...