also: のあわせいくさ
noun
open-field battle; battle in the open
Refers to a military engagement fought on an open field, as opposed to a siege or ambush. The word is somewhat literary and evokes pre-modern warfare.
戦国時代の野合戦では、鉄砲が重要な役割を果たした。
In open-field battles of the Warring States period, firearms played an important role.
この小説には、大規模な野合戦の場面が何度も出てくる。
This novel features several large-scale open-field battle scenes.
Compound of 野 (の, 'field') and 合戦 (かっせん, 'battle'). The reading のがっせん is a standard compound reading; のあわせいくさ is an older, less common reading.