The Trịnh–Nguyễn Lords were two rival ruling houses that divided Vietnam between roughly 1600 and the 1770s. The Trịnh governed the north (Đàng Ngoài, or Tonkin) and the Nguyễn the south (Đàng Trong), each ruling in the name of a powerless Lê emperor. Blocked past the fortified Gianh frontier, the Nguyễn pushed south instead, taking Cham lands by conquest, settlement, and tributary overlordship and entering the Khmer Mekong through migration and diplomacy as much as force, leaving Champa a shrinking vassal annexed only in the next century. The Tây Sơn rebellion destroyed both houses, and in 1802 the Nguyễn heir Nguyễn Ánh reunified the country and founded the Nguyễn dynasty.
For nearly a hundred years, two armies faced each other across a wall in central Vietnam and mostly did not fight. The Trịnh in the north and the Nguyễn in the south had fought seven bloody campaigns to a standstill, then simply stopped, each ruling its own half while bowing to the same emperor neither obeyed. Blocked from marching north, the southern lords turned the other way, pressing south into Cham and Khmer country by conquest and settlement but also by migration and tributary bargains that left Champa a shrunken vassal for another century. The long, thin map of Vietnam we know today was drawn less by unity than by a stalemate that had nowhere to go but south.