Beyond the Euphrates lived the fierce Parthians, one enemy the Romans were never able to conquer. Invasions by these fierce warriors plus barbarian tribes like the Huns, Vandals, Goths, and Visigoths devastated Rome. The cavalry charges of these savages simply overwhelmed the Roman legions who relied primarily on infantry. For some reason, they add details that John never mentions, namely, the horsemen are visible only to believers, are ghostly in nature and fly. That is what they repeatedly accuse non-premillennialists of doing.
History shows that is exactly what happened.
The two witnesses are surely Moses and Elijah who in turn represent the Law and the Prophets of Old Testament Scripture or Judaism. This interpretation of the vision woul relate it the church's survival of the Roman persecution and triumph over it as it preached the gospel. Many interpreters believe John is making use of Roman myth concerning Nero. Under this evil Caesar in the 60's A.D., Christians in the city of Rome itself were severely persecuted and the apostles Paul and Peter were marytred. After Nero's death, it was widely believed by the superstitious Romans that he would be reincarnated and return to attack Rome.