The Mongol Invasions of Japan
Kublai Khan, having taken China, sent two invasion fleets against Japan — the second among the largest naval operations before the twentieth century. Both were wrecked by typhoons while at anchor. The Japanese called the storms kamikaze, divine wind, and drew from them a conviction that the islands were under supernatural protection, a belief that would be invoked again 660 years later. The samurai who repelled the invasions bankrupted the shogunate that summoned them: there was no conquered land to pay them with, and the resentment helped bring it down.
An environmental shift (drought, cooling) strains food and water systems, amplifying every other tension at once.