front line 
noun (usually the front line)
1The military line or part of an army that is closest to the enemy: [as modifier]: the front-line troops...
- No longer were troops crammed into front-line trenches to provide easy targets for enemy artillery.
- Vehicle-mounted and manpack amplifiers were deployed in many theatres by front-line troops.
- The logistics of getting kit first to the Gulf and then to front-line troops like Sgt Roberts was also badly flawed, he said.
1.1The most important or influential position in a debate or movement: it is doctors who are on the front line of the euthanasia debate...
- The tired politics of the 1980s have little influence on the women in the front line of the movement today.
- The experience that Luca is slowly gathering, will help him fight for the front line positions at the next races.
- Is something changing on the front line of social movements and workers' struggles in France?