1. He is using theories on the frontier of astrophysics.

2. He wrote a book descriptive of the frontier provinces.

3. The incident on the frontier touched off a major war.

4. Lille is close to the frontier between France and Belgium.

5. Troops established a road block on the frontier.

6. They lived in a town close to the frontier.

7. There was an army checkpoint at the frontier.

8. There's a lot of smuggling across this frontier.

9. They massed well - equipped troops on the frontier.

10. The frontier has worked him over inside.

11. Troops are massing on the frontier.

12. Many people travelling across the frontier were illegal immigrants.

13. They were forced to retreat back over the frontier.

14. Space is the final frontier for us to explore.

15. They lined the north-western frontier with soldiers.

16. Until very recently Texas was an unsettled frontier.

17. The village straddles the frontier.

18. The rebels control the frontier and the surrounding area.

19. They didn't have permission to cross the frontier.

20. The frontier was pushed westward.

21. The frontier is patrolled by border guards.

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22. Beyond the frontier lay very real dangers.

23. The frontier station was starved for food and water.

24. It wasn't difficult then to cross the frontier.

25. The frontier town boomed when gold was discovered nearby.

26. Our car was turned back at the frontier.

27. The geographical position of the frontier fluctuated with the fortunes of war .

28. The army crossed the frontier in the middle of the night.

29. Neither country would guarantee the integrity of their common frontier.

30. There have been large - scale military manoeuvres on the frontier.