Why is the subjunctive used in this sentence?

[spqr]

Pyrrhus, Fabricii virtutem admiratus, illi secreto quartam etiam regni sui partem obtulit si patriam desereret.

[bedwere]

It's the protasis of a conditional clause in indirect speech. It must be subjunctive.

[Constantinus Philo]

Can we say: si patriam deserturus esset? I think it would be wrong but I don't know why.

[spqr]

The sentence does not look like the usual form of indirect speech to me.

[Hylander]

It's subjunctive because it implicitly represents what Pyrrhus said to Fabricius: "I am offering you a quarter of my kingdom if you desert." A speech act is implicit in obtulit.

I don't think deserturus esset, representing a future indicative verb in direct speech, would necessarily be wrong, but see Allen & Greenough 468, 516.2.a, Note, and 516.2.f:

http://www.perseus.tufts.edu/hopper/tex ... 99.04.0001

http://www.perseus.tufts.edu/hopper/tex ... 99.04.0001

[spqr]

Is this sentence an example of what some refer to as informal indirect discourse?

[Constantinus Philo]

Implied oratio obliqua

[spqr]

Ah yes, implied oratio obliqua. Thank you Constinus

[talus]

"Pyrrhus, having admired the manly virtue of Fabricius, proffered to him in secret, even a fourth part of his realm, if he would desert his fatherland."
[Note to the side: secreto is an adverbial use of an adjective to predicate an attribute to a person in the
specific context only. The text is not claiming that Fabricius is a secretive fellow as a matter of character but that he is secretive in this instance. Called by some "the adverbial use of an adjective as secondary predicate."]
We are used to seeing the imperfect subjunctive in contrary-to-fact conditionals. However the imperfect subjunctive can also be used of a possible future event (one that could actually come to fruition) that is future to a past situation. From Allen and Greenough 516, f:
"A future conditional is frequently thrown back into past time, without implying that it is contrary to fact." A&G give this example from Caesar:
non poterat, nisi decertare vellet
"He was not able, unless he wished to fight".