In an alternate second century where Romans rule a swathe of North America as far as the Dakotas, Titus Papinianus meets the Lakota chief Aquila who first teaches him a new way to fight the hoards from Asia and then leads him on adventures (including the time-traveling Central Dimension Patrol Authority) from modern-day Mexico to China.
I understood very little of what he was saying, but he went on to say that he was from the far future and that they had come in search of certain criminals who had to be brought to trial, who were guilty of attempting to tamper with the past. . . .

Variants

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  1. Aquila by S. P. Somtow, in Asimov’s Science Fiction, April 1982.
    2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . written by S. P. Somtow