The Japanese defenders recognized the possibility of a renewed invasion, and began construction of a great stone barrier around Hakata Bay in 1276. Completed in 1277, this wall stretched for 20 kilometers around the border of the bay. This would later serve as a strong defensive point against the Mongols. The Mongols attempted to settle matters in a diplomatic way from 1275 to 1279, but every envoy sent to Japan was executed. This set the stage for one of the most famous engagements in Japanese history.
When did Japan begin building the Hakata Bay barrier? (If the question is unanswerable, say "unanswerable")
1276