"The LORD spake also unto me again, saying, Forasmuch as this people refuseth the waters of Shiloah that go softly, and rejoice in Rezin and Remaliah's son; Now therefore, behold, the Lord bringeth up upon them the waters of the river, strong and many, even the king of Assyria, and all his glory: and he shall come up over all his channels, and go over all his banks: And he shall pass through Judah; he shall overflow and go over, he shall reach even to the neck; and the stretching out of his wings shall fill the breadth of thy land, O Immanuel." (Isaiah 8:5-8)
The name Shiloah means sent. A small spring erupting on the east side of Jerusalem was eventually channeled into the city. Flowing down from near the temple and terminating into a man-made pool at the bottom of the city, the waters of Shiloah are those referred to by Jesus as "the pool of Siloam (John 9:7)."
In this passage, the Lord uses this gently flowing spring to represent the peace and security of a kingdom whose trust is in the LORD of hosts. In contrast, Assyria is represented by an overflowing river which wreaks havoc on the surrounding landscape. Ahaz and his people must choose between the life-giving stream of God and the life-threatening river of the Assyrians.
This people may be in reference to northern Israel who had placed such confidence in Pekah and his ally Rezin; however, context would seem to indicate that God had in mind the people of Jerusalem when He made this statement. The small, weak province of Judah had become enamored with the seemingly great power and security of its northern enemies. Allowing human logic to guide their thinking, they had convinced themselves that their salvation lie in an alliance with Tiglathpileser whose might exceeded that of their opponents. The Lord promised that the king of Assyria would not be content to simply deliver Judah and then leave. This destructive Assyrian river would reach well into the land of Judah and threaten even Jerusalem itself; however, the city of God's choosing would be delivered as is seen in Isaiah 37:36-37.
Isaiah ended this warning with the same promise given in 7:14 - "God with us." Even though Judah would nearly destroy herself through her own rebellious choices, God's mercy and faithfulness would once again prove to be the means of deliverance.
Eventually, God would bring full judgment upon Jerusalem through King Nebuchadnezzar of Babylon; however, God's longsuffering nature kept His people from experiencing the full impact of their foolishness during the ministry of Isaiah. The Lord is very gracious. By God's mercy, a Christian is often spared the full and immediate impact of his poor choices while the Lord patiently chastens and directs those He loves (Revelation 3:19).