If the idea of negotiating were entertained at all, something would be gained; for each additional day of suffering and privation diminished the Roman strength, and brought nearer the moment of absolute and complete exhaustion. Moreover, a bridge might be at once commenced at some little distance, and might be pushed forward, so that, if the negotiations failed, there should be no great delay in following the Romans across the river.
It's a "first conditional", which is about something that could happen or not. That's called an open conditional.
For the moment, let's set it in the present:
"If the negotiations fail, there will be no great delay in following."
This represents what the Persians are thinking about negotiations.
Now, we have to set that conditional sentence in the past, to indicate what the Persians were thinking about negotiations, when the events were taking place:
"If the negotiations failed, there should/would be no great delay in following."
The use of the past tense "failed" and the word "should" (or "would") are required because this conditional sentence is set in past time. Although the negotiations didn't fail, they might have.
If you have a sentence set in the present time using past tense and "would", then that would indicate a second conditional, and an unlikely, or remote, "if" clause, but the example is set in the past.