A properly build and garisoned fort can hold for month, if not years. that was from ~8k BC, and that was true for WW2.
Yes, I agree, but as I said, these Sevastopol-like cases are exceptional: capital or strategic port, but not EVERY province. Most provinces should be occupied immediately.
Also, during real-world wars, there were so many cases when huge army arrives in some city and the city mayor just decides to surrender the city so it wouldn't be destroyed.