27: But if you refuse to send [them] out, I will strike your entire border with frogs.The frogs established the borders of Egypt.and did not go one step over the border into a neigboring country (Shmos Rabbah chapter 10). This implies that in areas where there was a border dispute, the frogs actually established what the border should be. This forced the Egyptians to admit where they were wrong and forced them to withdraw from areas that they claimed improperly.
Rabbi Sorotzkin points out that the fact that the hagaddah shows each plague as consisting of four (or five) individual components actually explains why this is not stated in the first palgue (blood). The water turned to blood as it entered the boundary of Egypt, but was restricted to the Nile River. Thus, the rest of the borders were not delineated by that plague. The frogs were thus the first plague to show the borders of Egypt.