main battle line; primary front
Refers to the principal area of military engagement or the most active front in a conflict. Can also be used figuratively for the main arena of a struggle or debate.
The allied forces attempted to break through the main battle line.
The primary front of the election campaign shifted to economic policy.
Compound of 主 (しゅ, 'main') and 戦線 (せんせん, 'battle line').