יחזקאל, פרק ל״ג, פסוק ג׳

A watchman stands at his post with one important duty: to spot danger as it happens and sound the alarm to save lives. The approaching threat is not a single weapon, but rather a wave of armed enemy troops marching forward with deadly intent [רש״י, רד״ק]. The critical moment to act depends on how the invasion unfolds. The watchman might spot the hostile forces just as they cross into the borders of the country [ביאור שטיינזלץ]. Alternatively, the realization might strike when the enemy has already arrived, gaining the upper hand and actively beginning to capture the city [מצודת דוד, מלבי״ם].

In either situation, the watchman must respond instantly by blowing a horn. This loud blast acts as a clear signal and an urgent alarm [מצודת דוד, ביאור שטיינזלץ]. The ultimate goal of this warning is to deliver a pressing message to the people. It gives them the crucial moment they need to protect themselves against the looming danger [מצודת דוד], or simply provides them the chance to run for their lives and escape the conquering army [מלבי״ם].