The order was given; now it is done. Aharon lifts the rod, strikes the Nile in full view of Pharaoh and his court, and the whole river turns (Exodus 7:20). Targum Pseudo-Jonathan on Exodus 7:20 lingers on the audience: in the sight of Pharoh, and in the sight of his servants.
Egypt's leadership watches its own lifeline die. This is not a private miracle. Moses does not touch the water himself — Aharon does. The meturgeman, here and in the following verses, is already preparing us for the moral that the Nile is too holy to Moses' own story for him to strike it. But at this moment, all that matters is the crowd.
The servants see. The king sees. The riverbank is crowded with witnesses. When the water shifts color, it does so in front of the very men whose power depends on believing Egypt's gods are in charge of the water. Egypt's theology dies on that bank before any Egyptian does.
The takeaway: God stages public signs for a reason. A miracle performed in secret can be denied tomorrow. A miracle performed in front of a royal retinue cannot. The Exodus begins not with a whisper but with a witnessed demonstration that the Nile answers to Someone greater than the king who worshiped it.