take the King's/Queen's shilling
old-fashioned To fight in a war in the name of England and its King (or Queen, depending on the ruling monarch of the time). Many Irish soldiers who took the King's shilling in World War I either died in battle or returned home to a country that now viewed them as traitors to the revolutionary cause.
take the King's (or Queen's) shilling
enlist as a soldier. British It was once the practice to pay a shilling to a man who enlisted as a soldier.