The kingdom-empire of Assyria plays a major role in the histories of the nations in Isaiah's time. They conquered the surrounding areas, many times, and would often have local rebellions. They destroyed the kingdom of Israel, but setup vassal kings to rule locally. After deporting the local populace, settlers came into conquered Israel. They had difficulties, and so imported some of the original people, who culturally became the Samaritans.
Assyria was basically a constant military threat, and while they ravaged the kingdom of Judah and the city of Jerusalem at times, they never managed to overrun it completely – the Lord preserved them.
Much of the history revolves around their campaigns. Through it all, the Lord speaks through Isaiah on how to remain a nation. In the end, Judah is the only kingdom left standing among its neighbors.
See Isaiah 10:12 for what the Lord will do to Assyria, and how he describes them.