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 were "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 "under existing Security Council resolutions on the basis that the Security Council considered that Iraq is in material breach of those resolutions" and "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". The question of whether these conditions were satisfied is controversial and unclear, since there was no further resolution which might have rendered the point clear. Alternatively, "under the right of self-defence if an armed attack by Iraq against the United Kingdom or one of its allies was reasonably believed to be imminent. This would not require any action by the Security Council." Greenwood acted as counsel for the government of the United Kingdom in relation to a number of cases in both domestic and international courts: the Ojdanic case in the International Criminal Tribunal for the Former Yugoslavia; Federal Republic of Yugoslavia v. United Kingdom in the International Court of Justice; the General Assembly request to the ICJ for an advisory opinion on the Palestinian wall ; R v. Immigration Officer at Prague Airport and others; and R v. the Secretary of State for Foreign and Commonwealth Affairs and others. Notable appearances include:
Libya v. United Kingdom ICJ Reps., 1992, p. 3; ICJ Reps. 1998, p. 3
Case concerning Legality of Use of Force ICJ Reps, 1999
R. v. Bow Street Magistrates, ex parte Pinochet 1 AC 147, 3 WLR 1456 and 1 AC 147, 2 WLR 827 1 WLR 188
Greenwood was elected as a judge of the International Court of Justice in November 2008 and served from 2009 to 2018. Greenwood's direct predecessor as a judge from the United Kingdom was Rosalyn Higgins and, although there is no rule allocating seats, the one held by them was kept by judges from the UK since the founding of the ICJ in 1946. In November 2017, the seats of Greenwood and four other judges were up for election. Nawaf Salam from Lebanon surprisingly contested and won the seat kept previously by Indian judge Dalveer Bhandari. India in turn had Bhandari contest the seat previously held by Greenwood and after multiple rounds of voting the United Kingdom decided to withdraw Greenwood's application. This is the second time a permanent member of the United Nations Security Council has no judge on the ICJ and first time a permanent member lost the majority vote in the UN general assembly.