Romans were always sensitive to the controllability of any territory that abutted their empire. What on earth would they have made of Afghanistan? Let alone its army?
Rex sociusque et amicus, ‘king, ally and friend’ was the honorific term applied to the ruler of people on the edge of their empire who agreed to come on board. The relationship was a delicate quid pro quo: Rome ensured that their new best friend remained securely in power, as long as he had a grip on his people, remained loyal to Rome and jumped when asked. The push-pull between Rome and the Parthian empire over Armenia offers a good example, both sides keen to have ‘their man’ in charge and make it look as if they were in control—for propaganda purposes if for nothing else—but without actually threatening the peace.
Economics came strongly into it. The point is that Roman soldiers living on the borders needed supplying, and it did not matter much where supplies came from. Further, such exchange helped to cement relationships on both sides. So networks of highly-respected ‘friends’ were in place even across what looks like the natural barrier of the Rhine-Danube. Another advantage was maintaining military strength. There was a good living to be made in the Roman army, and Romans knew the fighting worth of other armies, especially if they were German. But there was no point in playing these games if the stability of such kingdoms could not be relied upon or it had nothing to offer. It was then a matter of cutting losses (Scotland is a good example) or moving in full time.
President Karzai barely controls Kabul, let alone the country. An ‘economy’ hardly exists. And even if the West could bring prosperity to such a poor region, what would that guarantee? As for training up an army in a congenitally unstable region in the hope that it will remain on ‘our’ side when we are gone, Romans would have thought us deranged. Had they wanted a local army, they would have drafted Afghans, Gurhka-style, into the British army and sent them for training far away (as they often did to captives). As it is, we are merely creating mercenaries for hire by anyone who will want them.