What's that "id" doing?

[rutski]

I have the following passage from Livy to memorize and read in front of my class. Though not strictly required, I am naturally translating it for my own benefit (hard to memorize something if you don't know what it means!). The passage is given below. I've also provided a note of context related to the source so that the Latin will make more sense.

Livy, Ab Urbe Conditā XXII.39.11

The historian records a speech by Fabius Maximus, one of the heroes of the second Punic war, to Aemilius Paulus (consul 216 B.c.E.), who is about to set off to meet Hannibal on Roman soil. Fabius summarizes his view of the strategic situation. Aemilius later dies in combat near the Italian town of Cannae:

in Italiā bellum gerimus, in sāde ac solō nostrō; omnia circi plēna civium ac sociōrum sunt; armīs, virīs, equīs, commeātibus iuvant iuvābuntque, --- id iam fieī documentum in adversīs rēbus nostrīs dedērunt... Hannibal contrā in aliēnā, in hostīlī est terrā inter omnia inimīca īnfestaque, procul ab domō, ab patriā; neque illī terrā neque marī est pāx; nūllae eum urbēs accipiunt, nūlla moenia; nihil usquam suī videt...

At the moment I've gotten this far in my translation:

We wage war in Italy, in our own seat and in our own soil; on all sides everything is full of citizens, and allies; they help and will help us with arms, men, horses, and convoys --- id iam...

At that point I get minorly stuck. I think the next bit goes something like "--- they have already given proof of loyalty in our adverse matters" The thing I can't quite make sense of is what that "id" is doing there in "id iam"

Many thanks in advance,
-Patrick

[adrianus]

id [quod facunt] jam fidei documentum...dederunt = that [which they do] already they have...as a proof of...

[Qimmik]

Doesn't id just modify documentum? "they have already given that proof of their loyalty in our adversity. . . ", namely, their assistance with arms, manpower, etc.

[adrianus]

Verè ambiguum est, ut opinor.
You can have it both ways, I think.