Life and work as a soldier — the fighting, the training, the marching, the waiting, the whole trade taken together. 'He spent twenty years soldiering' means twenty years living that life. It is the -ing form of the verb 'to soldier', and it is used as a noun for the occupation, rather like 'teaching' or 'farming'. You meet it most in war memoirs, history and older writing, where a sentence like 'soldiering in the desert was hard' covers everything from the heat to the boredom to the fear. Two things worth knowing. First, the phrase 'soldier on' is far more common in everyday English and means something else: to keep going through difficulty without complaining — 'she was ill but soldiered on'. Second, in an older industrial sense, 'soldiering' meant workers deliberately working slowly, doing the least they could get away with; you will find this in books about factory management, and it survives mainly there. The ordinary meaning, though, is the first one: the job of being a soldier.

當兵、從軍的生活（整個軍旅生涯與工作）。中文常說「軍旅生涯」「當兵那段日子」。注意日常更常見的是片語 soldier on＝硬撐下去、咬牙繼續。