(d)action by an enemy or action taken in combating an enemy (whether with or without a declaration of war),and thereafter, at any time before the expiry of three years from the end of such tax year, the business of such undertaking is re-established, re-constructed or revived by the assessee, the condition referred to in sub-section (3)(a) shall not apply to such undertaking which is so re-established, reconstructed or revived.