"The people who walked in darkness have seen a great light." Isaiah 9:2
Because Isaiah 8 has referred to both the northerners and southerners, to Israel and Judah, it seems we have a hint of the reunification of Israel here in 9:2. More than that, the message is preached to the "Galilee of the Gentiles" and so the hint goes much further, to the reunification of all men, of Jew and Gentile. This will be the glorious work of the reconciliation of the world to God and to one another that Christ accomplished on the cross.
When people are walking in darkness, when there is strife between husband and wife, or between nations, the answer is the light of the gospel, the presence and zeal of Jesus, His grace and forgiveness, His law and duty, His justice and mercy.