Refers to the visible flames of a fire, often in the context of a spreading fire or conflagration. Common in news reports and literary descriptions.
Idiomatic use likening an attack or aggressive action to spreading flames. Often used in phrases like 火の手を上げる (to launch an attack) or 火の手が上がる (an attack breaks out).
The flames spread to the neighboring house.
When the firefighters arrived, the flames were already rising.
The enemy forces finally launched an attack.
The debate heated up, and he emotionally went on the offensive.
Compound of 火 (hi, 'fire') + の (no, possessive particle) + 手 (te, 'hand'), literally 'hand of fire'. The figurative extension to 'attack' likely comes from the image of flames reaching out aggressively.