Jamieson, Fausset and Brown on Luke 21:20-21

20-21. by armies--encamped armies, that is, besieged: "the abomination of desolation" (meaning the Roman ensigns, as the symbols of an idolatrous, pagan, unclean power) "spoken of by Daniel the prophet" (Da 9:27) "standing where it ought not" (Mr 13:14). "Whoso readeth [that prophecy] let him understand" (Mt 24:15).

Then ... flee, &c.--Eusebius says the Christians fled to Pella, at the north extremity of Perea, being "prophetically directed"; perhaps by some prophetic intimation still more explicit than this, which still would be their chart.