Korea's stock market just had one of its worst crashes ever, and the trigger came from America — bad news for Korean investors, and a warning about how fragile top-heavy markets are.

• Chip stocks sold off in the US in late June as investors got nervous ahead of Micron's earnings.
• Korea's market fell much harder because it is basically two stocks: Samsung and SK Hynix.
• Both fell more than 9% in a single day, then the whole KOSPI dropped over 10% on what Koreans now call Black Tuesday.
• A 10% one-day drop would be the third worst day in US market history, behind only 1987 and the Great Depression.

Outlook: Korea's market stays hostage to the AI chip trade, so more US chip weakness means more pain in Seoul.