The beacon fire blazed and the whole frontier knew the enemy had come.
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烽 is the signal fire lit on watchtowers to warn of invasion, mainly in 烽火. The phrase 烽火连天 - 'beacon fires linking the sky' - is a stock image for all-out war engulfing the land.
The fire radical on the left, a pictograph of rising flames. It directly supplies the meaning: a warning fire lit on a tower. It groups 烽 with 灯, 烟, and 炬.