the front line
noun [ S ] uk/ˌfrʌnt ˈlaɪn/ us/ˌfrʌnt ˈlaɪn/
（战争）前线
a place where opposing armies face each other in war and where fighting happens
Tens of thousands of soldiers died on the front line. 数以万计的士兵在前线阵亡。
More front-line troops will be flown to the battle zone over the next few days. 今后几天将会有更多的前线部队被调往交战区域。
Idiom
be in the front lineidiom 处于第一线
to be in an important position where you have influence, but where you are likely to be criticized or attacked
Many social workers are in the front line of racial tension. 很多社会工作者都处于解决种族紧张局面的第一线。