expression, ichidan verb
to open hostilities; to be at war
Archaic literary expression meaning to engage in armed conflict. Used in historical or formal contexts.
両国はついに干戈を交えるに至った。
The two countries finally came to open hostilities.
Modern general verb for fighting or waging war, while 干戈を交える is an archaic, formal expression.
干 (shield) and 戈 (halberd) are ancient weapons; the phrase literally means 'to cross shields and halberds', i.e., to engage in battle. The exact historical origin is uncertain, but it is a classical Chinese-derived expression.