Article 79 CISG
(1) A party is not liable for a failure to perform any of his obligations if he proves that the failure was due to an impediment beyond his control and that he could not reasonably be expected to have taken the impediment into account at the time of the conclusion of the contract or to have avoided or overcome it or its consequences.
(2) If the party's failure is due to the failure by a third person whom he has engaged to perform the whole or a part of the contract, that
party is exempt from liability only if:
(a) he is exempt under the preceding paragraph; and
(b) the person whom he has so engaged would be so exempt if the provisions of that paragraph were applied to him.
(3) The exemption provided by this article has effect for the period during which the impediment exists.
(4) The party who fails to perform must give notice to the other party of the impediment and its effect on his ability to perform. If the notice is not received by the other party within a reasonable time after the party who fails to perform knew or ought to have known of the impediment, he is liable for damages resulting from such non-receipt.
(5) Nothing in this article prevents either party from exercising any right other than to claim damages under this Convention.