Source: J.J. Collins, "The Sibylline Oracles," Jewish Writings of the Second Temple Period, ed. Michael E. Stone (Philadelphia: Fortress, 1984) 374.
"The assumption of Geffcken and Rzach that the conquest of Mesopotamia could only be prophesied after the fact is not justified. The Parthians were a menance to Roman power in the East in the first century B.C.E. and their subjection might well be prophesied by any one sympathetic to Rome."
This quote interested me because it talks about prophecy-after-the-fact--"prophecy" that "predicts" events that already occurred. Liberal scholars believe that chapters of Daniel fit into this category: events that were occurring in the second century B.C.E. were said to be prophesied centuries before by an earlier figure, Daniel.
According to Collins here, the prediction of the Sibylline Oracles that Mesopotamia (Parthia) would fall was not prophecy after the fact. It could have been wishful thinking, like the false prophets telling Ahab that he would win the battle, or those who told Judah that Babylon wouldn't conquer her, notwithstanding her sins.
It could have been a realistic forecast of what would happen, based on the situation at the time. Some have asserted that there are biblical prophecies that fall into this category. I read a liberal Christian article a while back, and it tried to explain away the prophecies that did not come to pass. For instance, Ezekiel predicted that Tyre and Egypt would be destroyed by Babylon, but that didn't exactly happen. The author of the article states that the prophet was interpreting the events of his time in light of his knowledge of God's will--how he believed God would act. He was like a religious pundit, commenting on world events.
I have a hard time seeing all of the biblical prophecies as statements of what was likely to happen, for there are some of them that are out-of-the-ordinary. When Isaiah said that Syria and Israel would fall rather than conquer Judah, that looked pretty unlikely! Isaiah even had to give Ahaz a sign--his prophecy looked that unbelievable! But prophecy isn't always a prediction based on where the trends seem to be going, for God can work in unexpected ways.