Jamieson, Fausset and Brown on Daniel 10:20

20. Knowest thou wherefore--The angel asks, after Daniel had recovered from his fright, whether he has understood what was revealed (Da 10:13). On Daniel, by his silence, intimating that he did understand, the angel declares he will return to renew the fight with the evil angel, the prince of Persia. This points to new difficulties to the Jews' restoration which would arise in the Persian court, but which would be counteracted by God, through the ministry of angels.

prince of Grecia shall come--Alexander the Great, who conquered Persia, and favored the Jews [Calvin]. Rather, as the prince of Persia is an angel, representing the hostile world power, so the prince of Grecia is a fresh angelic adversary, representing Greece. When I am gone forth from conquering the Persian foe, a fresh one starts up, namely, the world power that succeeds Persia, Greece; Antiochus Epiphanes, and his antitype Antichrist, but him, too, with the help of Michael, Israel's champion, I shall overcome [Gejer].