Yeah. Given that he SPECIFICALLY stated 15th century Spain... Indeed, the 15th century Spanish army...
Well, there's no getting around this. The Romans get slaughtered. Slaughtered. The Spainards kill the lot of them, and then pose the corpses for a laugh.
Well, there is just a slight problem.
How do you transport an ARMY from Spain to the Americas?
I define an army here as having at least several thousand (European) soldiers. Since cavalry, crossbowmen, pike men, cannons etc. were already mentioned here in this thread as being part of the supposed Spanish army.
It could be done if the build-up happens gradually and if you involve the Spanish Crown directly. And it assumes that the Spanish Crown pays for it and has the resources (money, ships, soldiers) available. No wars, no threats in Europe and the Mediterranean to divert their attention.
But then it probably won´t happen in the 1520s or 1530s. We´d be talking about a 16th century army then.
Both Cortés and Pizarro in OTL commanded only a few hundred soldiers in the 1520s and 1530s. With only a very few of them actually cavalry.
And they had a few other advantages too. Meeting:
- natives who weren´t sure if these white skinned people from overseas weren´t Gods.
- natives who´d never seen horses.
- natives who´d never seen iron / steel weapons and armor.
- natives who´d never seen - even slow firing and short range - arquebuses and small caliber cannons.
- natives who weren´t as politically "sophisticated" as the Spanish. Do you think a Roman Emperor with only a small guard would enter a town occupied by the Spanish (Atahualpa) or let them enter the capital (Moctezuma II) and take him as a hostage?
- natives (at least in Mexico) which were already experiencing European diseases (smallpox).
- Aztec and Incan armies which also seem to be very top-down? Kill the commander of the army and the army itself stops attacking. Seem to have happened a few times? I doubt that killing only the legate of a legion would have the same effect?
With the exception of gunpowder weapons and better armor (not covering the whole body though) all the other advantages have vanished against a Roman legion.
Now given that Unconsensual mentioned the TL.....
There might be one additional advantage plus one additional disadvantage?
Possible disadvantage:
After 1500 years in the Americas how "Roman" are the Romans there? Concerning Roman institutions, Roman army structures and the like? Citizenship laws? How many old texts did survive over the years? Or knowledge? Road building, aqueducts, mining, ship building?
Better said - given the limited size of the Roman expedition - how much of Roman culture and influence did survive?
There´s a difference between - say - an Aztec Empire with iron weapons and some upper class families citing Roman origins and an Empire - while mostly native after 1500 years - still following "somewhat" Roman traditions and culture.
The first one probably still gets conquered by Spain, it just takes longer. The second one probably not.
Possible advantage:
Difficult to believe that in 1500 years the "American Romans" didn´t improve their metallurgy at all?
Likewise difficult to believe that after 1500 years the "American Romans" won´t have larger faster horses too?
Bottom lime:
- Difficult to believe that Spain in the late 15th / early 16th century could send an army to the Americas
- The conquistadors (Cortés, Pizarro) with their gun-powder weapons might, just might win a first battle against a Roman Legion. Given their small numbers though they´ll lose the second battle against two Roman Legions.