何但 is a literary conjunction meaning 'how could it be only...?' or 'not to mention...' — it introduces an emphatic contrast, implying that if the first (lesser or more obvious) thing is true, then the second (greater or less obvious) thing must certainly be true as well. The character 何 (hé) functions here as an interrogative adverb meaning 'how' or 'why', while 但 (dàn) means 'only' or 'merely'; together, they form a rhetorical structure that negates limitation and signals escalation.

This expression appears almost exclusively in formal writing, classical texts, or highly stylized modern prose — such as historical essays, political speeches, or literary criticism. It is not used in everyday conversation. Its tone is elevated and persuasive, often employed to strengthen an argument by extending its scope beyond the immediately stated point. Learners should recognize it as a fixed literary pattern rather than a productive phrase for active use.