They recruited soldiers in the city in preparation for the campaign.
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募 is primarily a written/classical character. In modern Chinese it almost always appears in compounds: 募捐 (to solicit donations), 募款 (to raise money), 募兵 (to recruit soldiers).
Strength radical at the bottom, the indexing component. Pictograph of a plough or flexed arm. It marks 募 as effortful gathering - recruiting people or raising money takes force - and groups it with 努 strive, 勤 diligent, 助 help, 加 add.
Top 莫 supplies the sound with a regular shift mò to mù (lip-rounding group). The same phonetic powers 慕 admire, 暮 dusk, 墓 grave, 模 model - a tight mu family. Recognising 莫 on top predicts any of these readings.
