1206 AD Mongolia to 27 BC

  Interesting situation.  This is as the Latter Han are starting to reform their empire so its still weaker than a bit later although possibly stronger than OTL China that the Mongol's faced.  Later the Hun are supposed to have defeated the forebears of the Huns so a Mongol victory isn't certain but its probably very likely.  How long it would take and what bonuses the Huns might gain from it could be important as OTL they gained a fair bit of knowledge on siege equipment from their Chinese and other settled subjects.  Also with the different political and cultural encounters that might cause some confusion. 

  How long it would take the Mongols to expand westward I don't know, especially if here the Mongols concentrate 1st on overrunning all of China rather than only the northern part.  Although IIRC the Han control over southern China was somewhat less strong than in later centuries as there were far more non-Han populations in the region.

  One big difference as they do come west is that there will be a huge cultural difference, especially in terms of religion.  Other than scattered Jewish populations the only monotheistic population is the Persian Zoroastrians.  No Christians or Muslims in the world their now in, although there were some Nestorians  in the Mongol tribes so their going to have an interesting few questions to ask themselves when they realise their alive in the period just before Jesus was born.

  Assuming that the Mongol's don't get tied up in China too long and get rolling they will run into and probably defeat the just beginning Kushan empire although that might also divert some attention into India.  The Parthians are likely to put up a fight but probably go down as well.  To the norther I'm not sure of the status of the western steppes but tribes there are probably less developed and hence less strong than OTL.  Its pre-Rus so not sure what sort of societies are in place in what's now European Russian.

  In terms of Rome a lot depends on when the Mongols reach them, what information both sides have against the other and also what butterflies have occurred in Rome.  Most especially how stable and powerful the government is.  If its under a Nero or Caligula or undergoing a succession war then things could go very badly for it.  If under a strong emperor who doesn't get killed in the initial encounters then the empire would have a decent chance although it could lose much if not most of its eastern territories.  Rome at the time simply doesn't have massive experience against steppe cavalry armies other than some clashes with the Parthians.  Later it struggled with the Sassanids but against a well led Mongol force they going to have a very steep learning curve and it could take then a lot of bloody to find ways to counter the Huns.  If the latter have less siege capacity than OTL because their conquests are less developed then some places like Antioch and Damascus could hold out or at least put up a hell of a fight.  Its likely here that Egypt could fall as well.  The best Roman defensive line might be in the mountains of eastern Anatolia although the mongols were able to force their way through a lot of mountain ranges in their time.  I'm not sure what will happen here to the Jews in Palestine, which could depend on the circumstances of the time an the nature of the Mongol leader who conquers the region.

  They could be better off in Europe as with less developments the Mongols could struggle more to project power through the forests and other rough ground of northern Europe and with the region generally less populated and more marshy than later you could see less of a threat of hordes of refugees fleeing from the Mongols.  [Plus in this time period without the same religious issues of the 4th and 5th centuries the empire is more likely to welcome them as potential auxiliaries].  Also the Mongols tended not to be too successful at sea so if the Romans build up their fleets they could keep the Mongols out of Europe and Africa west of Egypt.

  Anyway initial thoughts but as always the devil is in the details and it would be impossible to predict realistically how things would develop.

  I think there's going to be a more developed silk road either way.  In fact its more likely if after a period of time the empire has survived and more friendly relations occur.  Not much scope for trade if the other end is a devastated wasteland.