Iran has announced that it will be sending a ship with humanitarian supplies to Yemen, departing the evening of May 10th. Many parts of the Yemeni conflict raise law of war questions, from the legality of the pan-Arab intervention to questions about the use of force and civilian casualties. The Iranian relief ship puts into focus the blockade maintained by Saudi Arabia and its allies, with logistical and intelligence support from the United States. Saudi Arabia imposed a blockade of Yemen’s ports from the start of the campaign. Since then, the humanitarian situation has become dire, according to many reports, with significant shortages of medicine, food and water. (Saudi Arabia also bombed the Sanaa airport to prevent Iranian relief planes from landing.) According to Oxfam, “there is no exit” for Yemen’s 10 million people, half of whom are already going hungry. Blockade is an entirely valid military tactic, which necessarily puts pressure on the civilian economy and well-being. However, there is a theory, which in recent years has attracted considerable support, that international law prohibits blockades in a non-international armed conflict (NIAC). This limitation on blockade has been discussed almost exclusively in connection with Israel’s blockade of Gaza. Assuming that the Yemeni conflict is a NIAC, as most observers seem to view it (a civil war with foreign assistance to both sides), the Saudi blockade raises the same questions as the Gaza blockade, as Tehran has gleefully noted. To be sure, considerable authority concludes that blockade is entirely permitted in NIACs. The Saudi blockade gives a good occasion to revisit the debate, which has thus far proceeded with an incomplete account of state practice. Israel’s blockade of Gaza appears to be the first one where said to be illegal because of the nature of the conflict. In the Gaza context, the illegality argument was based largely on what was said to be scanty affirmative precedent for such actions in such contexts, though a lack of precedents does not normally create a prohibition in international law. Though it was not mentioned in the extensive discussions of Israel’s Gaza policy, there is not only historical precedent, but also contemporary practice supporting NIAC blockades. In particular, Georgia’s blockade of the separatist Abkhazia region, which has been in effect since 2008. The details of the blockade are murky, in part because it has generated not only no international protest, but also no international interest. It is clear that the blockade has been used to interdict neutral vessels carrying non-military supplies. Indeed, the blockade is so well accepted, that the commentators on the legality of the Gaza blockade appear to have been entirely unaware of it. Then there is Sri Lanka’s blockade of Tamil-held areas during their decades-long civil war. Douglas Guilfoyle, the author of one of the major analyses of the legality of the Gaza blockade, dismissed the relevance of the Sri Lankan precedent: Most reported maritime interceptions appear to have occurred with Sri Lanka’s territorial sea or contiguous zone, ostensibly on suspicion the vessels were engaged in smuggling weapons or supplies… The practice certainly involved no assertion of rights against neutral vessels on the high seas. Unfortunately, this account appears to be mistaken on all major points. The blockade certainly applied to neutral ships carrying food and relief supplies, even under Red Cross emblem. Indeed, the blockade resulted in major shortages of basic necessities. The seizure Guilfoyle points to as being within the contiguous zone was, according to all other news accounts, well outside it (and was in any case after the cessation of hostilities and defeat of the Tamils). Nonetheless, the international community does not appear to even have questioned the legality of this blockade. In another precedent that has not factored into the NIAC-blockade discussion, Indonesia imposed a naval blockade on East Timor when it invaded the territory in 1975, according to accounts of the conflict. Despite fairly strong international condemnation of the invasion itself, I have not found specific criticism of the legality of the blockade. Incidentally, in 1992, a “peace ship” carrying activists, Western politicians, and a slew of journalists was turned back by the Indonesian navy after attempting to symbolically challenge that blockade. In that incident, the ship turned back of its own accord after Indonesian threats to open fire; despite the strong international focus on the incident at the time, no one suggested the illegality of such actions in a NIAC. There may be other recent state practice that has gone unnoticed as well. The episodes discussed here generated relatively little legal controversy – ironically, permissive precedent is most likely to go unnoticed. (The discussion’s of Israel’s blockade dwelt mostly on the United States blockade of Confederate ports in the Civil War and the France’s blockade of Algeria, rather than more current ones, no doubt because they attracted more attention, and better sourced in English and French publications than the Indonesian, Georgian and Sri Lankan measures.) The blockades discussed here, including the Saudi one, all appear to proceed without all of the formality of the a traditional international armed conflict blockade; for example, it is not clear that there were formal declarations, and the blockaded enemy does not seem to have been always been recognized as a belligerent. This suggests state practice supports a less legally restrictive blockade regime for NIACs. Thus if Riyadh and its allies are inclined to maintain the blockade, and intercept the Iranian relief ship, it has a strong legal basis. Of course, the Saudi blockade itself becomes part of the state practice on this issue, and on other blockade issues such as proportionality. One may have thought that, prior state practice to the contrary, Gaza suggested an interest by some states in changing the rules about blockade in NIACs. The Yemen blockade, in force since late March, has not been denounced as illegal, suggesting that no new rule is taking shape. In regards to the conduct of the blockade, it is interesting to note that Human Rights Watch today criticized the coalition’s conduct of the blockade, in particular urging for allowing in fuel. The report, which is well worth reading for more detail on the naval blockade, paints an absolutely catastrophic picture of the situation in Yemen, with much of the population facing death by hunger, water shortage and associated diseases. Interestingly, HRW does not challenge the legality of the blockade, or its apparently very narrow list of “free goods” (those permitted to pass the blockade after being subject to inspection). In particular, HRW does not call for the US or the UN to condemn the operation, as it has for other blockades. While HRW interestingly reports that the Saudi’s contraband list is not public (generally a legal problem for blockade), it also does not protest what appear to be its fairly comprehensive scope.