Who they were
The nineteenth king of Goguryeo, reigning 391–412. His reign name means “broad expander of territory,” and he is the only Korean ruler traditionally styled “the Great” alongside Sejong.
What they did
His campaigns pushed Goguryeo across Manchuria and the Liaodong region, and southward on the peninsula. In 400 he sent troops — tens of thousands by traditional count — to aid Silla against Wa (Japanese) and Gaya forces.
Legacy
His son King Jangsu erected the Gwanggaeto Stele at the capital (modern Ji’an, China) in 414 — the oldest major Korean inscription. Its passages about the Wa are read differently by Korean and Japanese historians. Under Gwanggaeto, Goguryeo reached its greatest extent, and he stands as a symbol of national strength in modern Korea.