From the very beginning, this relentless drive has put Ethiopia at odds with Egypt. The Renaissance Dam is on the Blue Nile, a tributary that contributes most of the water flowing into the Nile River, heightening concerns that it could threaten Egypt’s most vital natural resource. Fears of armed conflict surfaced during the brief tenure of Egypt’s former president, Mohamed Morsi, who said last year that “Egyptian blood” would substitute for every drop of lost water.But under Egypt’s current president, Abdel Fattah el-Sisi, the icy relationship between the two countries has begun to thaw. Ethiopia’s prime minister, Hailemariam Desalegn, and Mr. Sisi had a cordial first meeting in June, and water ministers from Ethiopia, Egypt and Sudan met for renewed discussions in late August. Egypt’s new foreign minister, Sameh Shoukry, set a diplomatic tone during a visit last month to the Ethiopian capital, Addis Ababa, declaring “a new phase of our relationship based on mutual understanding, mutual respect and a recognition that the Nile binds us.”