By the 16th century, after the glorious Hồng Đức Era, the Later Lê Dynasty weakened. Corrupt mandarins and a decaying court led to famine, rebellions, and fragmented allegiance among the people. In the court, power fell into the hands of the Mạc clan, a family that rose from the Cao Bằng region.
The Lê King was deposed, the land was divided, and the people called that era the "Mid Restoration Calamity."
But amidst the turmoil, two other forces arose: the Trịnh clan in the North and the Nguyễn clan in the South.
Both initially raised the banner to restore the Lê and defeat the Mạc, but soon, the flags of the same red color confronted each other, driven by ambition and divergent popular sentiment.
After the Mạc seized the throne, General Nguyễn Kim revived the Lê restoration banner, enthroning King Lê Trang Tông in Thanh Hóa.
Beside him was his son in law, Trịnh Kiểm, a shrewd, composed, and strategically gifted man. When Nguyễn Kim was poisoned, Trịnh Kiểm took over, using the pretext of supporting the Lê to seize complete military authority.
From then on, the Trịnh Lords began to rule de facto in the North, though they still honored the Lê King as the "nominal Son of Heaven." People called that period "Vua Lê Chúa Trịnh" (The Lê King The Trịnh Lord).
As the Trịnh clan's power grew, Nguyễn Hoàng, the youngest son of Nguyễn Kim, feared suspicion. He heeded the advice of Trạng Trình Nguyễn Bỉnh Khiêm, a man who saw deep into the human heart, who instructed him:
"Hoành Sơn nhất đái, vạn đại dung thân."
(A strip of the Hoành Sơn mountain range can shelter you for ten thousand generations.)
Nguyễn Hoàng thus asked to govern Thuận Hóa (present day Huế), and from there, expanded southward into Quảng Nam, Quảng Ngãi, and Quy Nhơn, laying the foundation for the prosperous Southern region. From this point, the nation of Đại Việt was divided into:
The Outer Region (Đàng Ngoài) under the Trịnh Lords,
The Inner Region (Đàng Trong) under the Nguyễn Lords.
The two halves spoke the same Vietnamese language and worshipped the same Dragon and Fairy ancestors, yet they pointed their spears at each other across the Gianh River.
From 1627 to 1672, the Trịnh Nguyễn clans clashed over ten times. Blood filled the Gianh River, and the Quảng Bình region became the border of its own people.
Cannons, warships, and firearms from the West were introduced. Both sides grew rich and powerful, but neither could achieve victory. The people were exhausted, a nation divided only to trample on its own shadow.
Yet, amidst the smoke and fire, there were still people whose hearts yearned North for reunification and South for peace.
Although the war continued intermittently, culture, scholarship, and commerce flourished. The Outer Region excelled in scholarship, while the Inner Region excelled in trade. Phố Hiến (Hưng Yên) and Hội An (Quảng Nam) became two international trading hubs, where Japanese, Chinese, and Westerners gathered.
The Inner Region opened its doors to the world, trading with Japan, Portugal, and the Netherlands. The Outer Region maintained its traditional rites, emphasized learning, and preserved Confucianism and the national academic spirit.
The two halves were like two different currents of the same river one strong in reason, the other deep in sentiment.
In the 17th 18th centuries, Nguyễn Phúc Chu, succeeding his ancestors, expanded the territory south, pioneering the Mekong Delta, establishing Gia Định Citadel, and creating the prosperous Southern land. He said:
"If my father opened the land, I shall sow literature, sow ethics, and sow the people's hearts."
The Việt people followed the Nguyễn Lords southward, crossing forests and wading rivers, carrying the belief that any land that held Việt blood was their homeland.
The war between the two regions lasted until the late 18th century, when the Lê Dynasty collapsed and the Tây Sơn uprising began. But before the flames consumed everything, the people of the two regions quietly prayed the same prayer:
"One nation, one land and river, please do not remain separated forever."
Two hundred years of bloodshed, yet the flame of patriotism never extinguished. For when the sword of a nation cuts into itself, that wound transforms into a lesson for all generations to come.