What is the origin of the idiom "battle line" and when to use it?

The origin of the idiom "battle line" can be traced back to military tactics and strategies employed in ancient warfare. The concept of organizing forces in a linear formation can be found in various historical periods and civilizations, including ancient Greece, Rome, and medieval Europe. The specific term "battle line" itself emerged as a descriptor for the position or alignment of troops facing the enemy during conflicts. Over time, the term has been adopted and adapted in different contexts beyond its military origins to represent divisions or confrontations in various arenas, such as politics, sports, or even metaphorical battles of ideas.