Why was the Berlin wall built in 1961?

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Across
  1. 1. Soviet demand for Western withdrawal from Berlin
  2. 3. Berlin split between East and West
  3. 6. Stopped the population drain
  4. 8. Wall built overnight in August
  5. 13. Wall separated relatives for decades
  6. 14. Loss of workers damaged the GDR
  7. 17. Skilled workers leaving East Germany for the West
  8. 18. Wall showed people wanted to flee communism
  9. 19. US President accepted the Wall as better than war
  10. 20. Wall became symbol of oppression
Down
  1. 2. East German leader demanded action
  2. 4. Famous crossing point between sectors
  3. 5. Wall strengthened communist authority
  4. 7. Wall deepened Cold War hostility
  5. 9. Millions fled through Berlin before 1961
  6. 10. West Berlin attracted East Germans
  7. 11. Berlin a major source of tension
  8. 12. America refused to accept East German legitimacy
  9. 15. Initial fence before concrete wall
  10. 16. Main purpose was to stop East Germans leaving