Why is Afghanistan known as the graveyard of empires?

Related questions

1. Who pays bride price?
2. Is Afghanistan rich?
3. Does Pakistan have oil?
4. Who shot Malala?
5. What religion is in Afghanistan?
6. Are Turks Caucasian?
7. Why Afghanistan is a poor country?
8. Was Afghanistan a Hindu country?
9. How many US soldiers died in Syria?
10. How many Taliban killed?

Tags

1. #battleground
2. #afghanistan
3. #great
4. #solely
5. #coined
6. #empires
7. #called
8. #graveyard
9. #perhaps
10. #because
11. #unclear
12. #better
13. #truth
14. #perished
15. #disputable