1)
Did Hashem command only now? The Aron and Kohanim already passed (verse 11)!
1.
Rashi (16): The verse tells now what Hashem commanded beforehand.
2.
Malbim (16): Hashem now commanded that they ascend from the west bank to the dry land. Since Hashem had commanded to stand in the Yarden, they could not ascend without permission.
