As one man vs To a man

as one man—(of a group of people) acting unanimously and at exactly the same time (also: as a man):

• As he uttered these words, the members of the convention rose as one man to their feet.

to a man —(also: to the last man)

1. (of a group of people) every person without exception:

• The inhabitants, almost to a man, had left the city.

2. (of combatants) keep fighting until none is left alive:

• The Persian army closed in on them, and the Spartans died fighting to a man.