If the soldiers are always required to issue warnings, 'any' is better. However, both of them sound a little strange. I would change it to: An army spokesman stressed that all the soldiers had been ordered to issue clear warnings before shooting.

Any answer could be correct; but some answers may not be correct. Any of the fruit in the basket could be bad; but some of the fruit will be good to eat. Any of us can make an occasional mistake; but some of us make mistakes all of the time. Any answer could be correct, but some of us will make a mistake in our answer. Some people may not have all of the answers, but at any given moment, it is true that many will offer correct answers.