God spoke to Abraham 8 times (Gen 12:1-3 & 7, Gen 13:14-17, Gen 15, Gen 17, Gen 18, Gen 21:12-13, Gen 22:2-18). God promised him the land of the Canaanites, and a nation of descendants that would live on that land. In Genesis 15:18-21, God specified the borders of the land as from the river of Egypt to the south to the Euphrates River to the east, and He listed 10 nations that Abraham's descendants would replace. Several times later the list of nations was given as 7 (Ex 3:8 & 17, Deut 7:1). God extended this promise to Abraham's son Isaac (Gen 26:2-5), to his son Jacob (Gen 28:13-15), and to their descendants the people of Israel. The land was also called the land which God sware to the patriarchs, and a land flowing with milk and honey.
God also spoke to Moses about this promise (Ex 3:8 & 17, Ex 6:8, Ex 13:5 & 11, Ex 32:13, Ex 33:1-3, Lev 14:34, Lev 25:38, Num 13:2 & 17, Num 32:32, Num 33:51, Num 34:2, Deut 1:7, Deut 7:1). The details of the the borders of Israel were explained in Numbers 34 which can be summarized as the river of Egypt to the south, the Mediterranean Sea to the west, the Sea of Galilee to the north, and the Jordan River to the Dead Sea to the east. Joshua distributed the land to the 12 tribes after he conquered the land (Josh 15-19). This included the land east of Jordan that 2 1/2 tribes inherited after it was taken from Sihon and Og (Num 21, Num 32, Josh 13, Josh 22).
We naturally think of the Jordan River as the border of Israel. It is the river the Israelites crossed on dry land to make their official entry into the promised land (Josh 3). However, the Euphrates River is mentioned 4 times as a border (Gen 15:18, Deut 1:7, Deut 11:24, Josh 1:4). God promised to enlarge Israel's borders (Ex 34:24, Deut 12:20). He put the condition of obeying His commandments to this promise in Deuteronomy 19:8-9. It appears that at some point during the reign of David this opportunity for enlarged borders was realized (2Sam 8:3, 1Chr 5:9, 1Chr 18:3, Jer 13, Jer 46:1-10, Jer 51:63).
The Euphrates River starts in Turkey and flows through Iraq to the Persian Gulf. The land of Iraq is the same land Babylon used to be on. It is the longest River in that area, and with the Tigris defines Mesopotamia, which means land between the rivers. It was one of the rivers that marked the area near the garden of Eden before the flood (Gen 2:14). This river will also play a part in the events of the tribulation period (Rev 9:14, Rev 16:12).
Thus, the Euphrates symbolizes for us an increased spiritual jurisdiction. We can be content with our border as the Jordan. That is part of God's promises to His people. We can be in the kingdom, having crossed the water of baptism into God's blessings (1Cor 10:1-4). We can be happy with good, or we can pursue the best of what God has for us, and seek to enlarge our borders to the fullness. It will require commitment and obedience. We can be saved and be happy with the Jordan, which is good; or we can seek the best and push our borders to Euphrates and experience the fullness. We can take the jurisdiction away from Sihon and Og, the enemies that rise against us to contain us, and enjoy greater victory. To do this, we must know that the Euphrates is ours to take. We can just assume that the Jordan is all there is for us, or we can dig deeper into the promises of God and find more. Israel only asked permission to pass through the land of Sihon en route to the Jordan (Num 21:21-35). They also had no intention of attacking Og. Sihon and Og attacked them. God gave Israel a victory they were not even looking for when they fought against those who tried to hinder and limit them. When we exercise our faith for the greater things, God will give us things we had not even realized are ours for the taking.