Greenwood is well known for the October 2002 legal opinion tendered to the British government, entitled "The Legality of Using Force Against Iraq". The legal opinion, which he signed in his capacity as a law professor, has been used to justify that the invasion by Britain, the United States and allied powers was sanctioned by the UN Security Council. However, the opinion was concluded in the month before the adoption of United Nations Security Council Resolution 1441 and the conclusion was stated to be dependent on one of three conditions being satisfied. These conditions (he said) were (1) "if the UN Security Council adopts a fresh resolution authorising military action against Iraq and any conditions set out in that resolution are met" – this did not happen; or (2) "under existing Security Council resolutions on the basis that the Security Council considered that (a) Iraq is in material breach of those resolutions" and (b) "that breach constitutes a threat to international peace and security in the Gulf area. This would not require a fresh Security Council authorisation of military action".