1 The incident was widely reported. See, e.g., “U.S. Demands Return of Drone Seized by Chinese Warship”, New York Times, 16 December 2016, available at: www.nytimes.com/2016/12/16/us/politics/us-underwater-drone-china.html.

2 Jane Perlez and Matthew Rosenberg, “China Agrees to Return Seized Drone, Ending Standoff, Pentagon Says”, New York Times, 17 December 2016, available at: www.nytimes.com/2016/12/17/world/asia/china-us-drone.html.

3 US Department of Defense (DoD), Statement by Pentagon Press Secretary Peter Cook on Incident in South China Sea, 16 December 2016 (Bowditch Statement), available at: www.defense.gov/News/News-Releases/News-Release-View/Article/1032611/statement-by-pentagon-press-secretary-peter-cook-on-incident-in-south-china-sea.

4 See, e.g., J. Perlez and M. Rosenberg, above note 2; Chris Buckley, “Chinese Navy Returns Seized Underwater Drone to U.S.”, New York Times, 20 December 2016, available at: www.nytimes.com/2016/12/20/world/asia/china-returns-us-drone.html.

5 For attempts at answering some of these questions, see, e.g., James Kraska and Raul “Pete” Pedrozo, “China's Capture of U.S. Underwater Drone Violates Law of the Sea”, Lawfare, 16 December 2016, available at: www.lawfareblog.com/chinas-capture-us-underwater-drone-violates-law-sea; Julian Ku, “The Nonexistent Legal Basis for China's Seizure of the U.S. Navy's Drone in the South China Sea”, Lawfare, 16 December 2016, available at: www.lawfareblog.com/nonexistent-legal-basis-chinas-seizure-us-navys-drone-south-china-sea.

6 For commentary on the debate surrounding unmanned aerial systems, see, e.g., Schmitt, Michael N., “Narrowing the International Law Divide: The Drone Debate Matures”, Yale Journal of International Law Online, Vol. 39, 2014Google Scholar. On the subject of autonomous weapons more broadly, the strength of feeling on one side of the controversy is demonstrated by the Campaign to Stop Killer Robots; see the “About Us” page on the Campaign's website, available at: www.stopkillerrobots.org/about-us/.

7 DoD, Unmanned Systems Integrated Roadmap FY2011–2036, 2011, p. 25, available at: www.acq.osd.mil/sts/docs/Unmanned%20Systems%20Integrated%20Roadmap%20FY2011-2036.pdf; US Department of the Navy, The Navy Unmanned Surface Vehicle (USV) Master Plan, 2007 (USV Master Plan), p. 1, available at: www.navy.mil/navydata/technology/usvmppr.pdf.

8 DoD, above note 7, p. 16.

9 Megan Eckstein, “Interview: Rear Admiral Robert Girrier on the Future of the Navy's Unmanned Systems”, USNI News, 12 October 2016, available at: https://news.usni.org/2016/10/12/22016.

10 See, e.g., DoD, Defense Science Board, Task Force Report: The Role of Autonomy in DoD Systems, 2012, pp. 17–18, available at: www3.nd.edu/~dhoward1/autonomy.pdf.

11 Scott Savitz et al., U.S. Navy Employment Options for Unmanned Surface Vehicles (USVs), 2013, p. 39, available at: www.rand.org/content/dam/rand/pubs/research_reports/RR300/RR384/RAND_RR384.pdf.

12 USV Master Plan, above note 7, p. 11.

13 Antoine Martin, Unmanned Maritime Systems: Global Review of Technology, Roadmaps, Roles, Challenges & Opportunities, and Predictions, undated, p. 5, available at: www.uvs-consulting.com/wp-content/uploads/2014/01/AUVSI-Aug-13-UMS-Global-Review-UVSC.pdf.

14 USV Master Plan, above note 7, p. 19.

15 Ibid., p. 18.

16 Ibid., pp. 23–24.

17 Ibid., pp. 34.

18 Combined Joint Operations from the Sea Centre of Excellence (CJOS/COE), Study (2009) for Maritime Unmanned Systems (MUS) in NATO, 2009, p. 32, available at: www.cjoscoe.org/images/MUS_in_NATO.pdf.

19 USV Master Plan, above note 7, p. 45.

20 Because of the curvature of the Earth's surface, the closer a sensor or transmitter is to the water, the smaller the radius over which it can establish and maintain a line of sight to the target.

21 USV Master Plan, above note 7, p. 48.

22 Bowditch Statement, above note 3.

23 Fastwave, Submission to Defence White Paper 2015: Unmanned Maritime Systems, 2015, available at: www.defence.gov.au/Whitepaper/docs/256-Fastwave.pdf.

24 Rick Stella, “Ghost Ship: Stepping Aboard the Sea Hunter, the Navy's Unmanned Drone Ship”, Digital Trends, 11 April 2016, available at: www.digitaltrends.com/cool-tech/darpa-officially-christens-the-actuv-in-portland/; Rachel Courtland, “DARPA's Self-Driving Submarine Hunter Steers Like a Human”, IEEE Spectrum, 7 April 2016, available at: http://spectrum.ieee.org/automaton/robotics/military-robots/darpa-actuv-self-driving-submarine-hunter-steers-like-a-human.

25 Arhur Villasanta, “DARPA's Sea Hunter Killer Drone Will Make China's Submarines More Vulnerable”, Yibada, 28 July 2016, available at: http://en.yibada.com/articles/146010/20160728/darpa-s-sea-hunter-killer-drone-will-make-china-submarines.htm.

26 US Department of the Navy, The Navy Unmanned Undersea Vehicle (UUV) Master Plan, 2004 (UUV Master Plan), p. 4, available at: www.navy.mil/navydata/technology/uuvmp.pdf.

27 DoD, above note 10, pp. 85–86.

28 UUV Master Plan, above note 26, p. 11; CJOS/COE, above note 18, p. 24.

29 UUV Master Plan, above note 26, p. 12.

30 CJOS/COE, above note 18, pp. 24–25.

31 DoD, Defense Science Board, Summer Study on Autonomy, 2016, pp. 61–62, available at: www.acq.osd.mil/dsb/reports/DSBSS15.pdf.

32 See, e.g., Bruce Dorminey, “How Bad Would It Be if the Russians Started Cutting Undersea Cables? Try Trillions in Damage”, Forbes, 2 November 2015, available at: www.forbes.com/sites/brucedorminey/2015/11/02/russian-navy-probing-u-s-undersea-communications-cables-in-new-global-threat/#6b625ac766b1.

33 DoD, above note 31, p. 43.

34 Kyle Mizokami, “Pentagon Confirms Russia Has a Submarine Nuke Delivery Drone”, Popular Mechanics, 8 December 2016, available at: www.popularmechanics.com/military/weapons/a24216/pentagon-confirm-russia-submarine-nuke/.

35 Kevin McCaney, “Boeing's New Autonomous UUV Can Run for Months at a Time”, Defense Systems, 14 March 2016, available at: https://defensesystems.com/articles/2016/03/14/boeing-echo-voyager-uuv.aspx.

36 United Nations Convention on the Law of the Sea (UNCLOS), 1833 UNTS 397, 10 December 1982.

37 Vienna Convention on the Law of Treaties (VCLT), 1155 UNTS 331, 23 May 1969, Art. 31(1)–(2).

38 UNCLOS, Art. 92(4)(b)–(c).

39 1962 Amendments to the 1954 International Convention for Prevention of Pollution of the Sea by Oil, 600 UNTS 332, 11 April 1962, Art. 1(1).

40 International Convention for Prevention of Pollution from Ships of 1973, as amended by the 1978 Protocol, 1340 UNTS 61, 184, 17 February 1978, Art. 2(4).

41 Protocol to the Convention on the Prevention of Marine Pollution by Dumping of Wastes or Other Matter, 36 ILM 1, 7 November 1996, Art. 1(6). See also Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, 1046 UNTS 138, 29 December 1972, Art. III(2).

42 Convention on the International Regulations for Preventing Collisions at Sea, 28 UST 3459, TIAS No. 8587, 1050 UNTS 16, 20 October 1972, Rule 3(a).

43 United Nations Convention on Conditions for Registration of Ships, UN Doc. TD/RS/CONF/19/Add.l, 7 February 1984 (not yet in force), Art. 2.

44 VCLT, Art. 31(3)(b).

45 For a contrary view, see, e.g., J. Kraska and R. Pedrozo, above note 5.

46 DoD, Office of the General Counsel, Law of War Manual, June 2015, updated December 2016 (DoD Manual), § 13.1.2.

47 Bowditch Statement, above note 3.

48 UNCLOS, Arts 18(2), 38(2), 53(3).

49 Ibid., Arts 19(2), 39(1), 54.

50 Ibid., Art. 19(2).

51 Ibid., Arts 39(1)(c), 54.

52 Ibid., Art. 20.

53 Convention No. VII relating to the Conversion of Merchant Ships into War-Ships, 18 October 1907, 205 Consol. TS 319, Arts 2–6.

54 See, e.g., Doswald-Beck, Louise (ed.), San Remo Manual on International Law Applicable to Armed Conflicts at Sea, Cambridge University Press, Cambridge, 1995CrossRefGoogle Scholar (San Remo Manual), para. 13(g).

55 UNCLOS, Art. 107.

56 Ibid., Art. 110.

57 Ibid., Art. 111(5).

58 Ibid., Art. 224.

59 Ibid., Arts 107, 111(5), 224. Similarly, Article 110(5) provides for the right of visit to be exercised by “other duly authorized ships or aircraft clearly marked and identifiable as being on government service”.

60 German Navy, Commander's Handbook: Legal Bases for the Operations of Naval Forces, SM 3, 2002, p. 45.

61 US Navy, US Marine Corps and US Coast Guard, The Commander's Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12/COMDTPUB P5800.7A, 2007 (US Commander's Handbook), para. 2.3.6.

62 The principle is reflected in the United Nations Convention on Jurisdictional Immunities of States and Their Property, UNGA Res. 59/38, 2 December 2004 (not yet in force). The Convention is based on draft articles adopted, with commentaries, by the International Law Commission (ILC). ILC, Report on the Work of Its Forty-Third Session, UN Doc. A/46/10, 1991, p. 13. As the ILC notes, “[l]egal theories abound as to the exact nature and basis of immunity. … Beyond or around the hard core of immunity, there appears to be a grey area in which opinions and existing case law and, indeed, legislation still vary.” Ibid., p. 23.

63 UNCLOS, Arts 2(3), 19(1), 21(1), 31, 34(2), 58(3), 87(1), 138, 293(1), 297(1)(b).

64 As an object, a UMS may generally be attacked so long as it meets the definition of a military objective: see San Remo Manual, above note 54, paras 40–41. To the extent that an enemy unmanned maritime system might be considered a vessel, even if it does not qualify as a warship, it is subject to capture and prize adjudication when outside neutral waters: Ibid., paras 135–138. On booty of war, see Henckaerts, Jean-Marie and Doswald-Beck, Louise (eds), Customary International Humanitarian Law, Vol. 1: Rules, Cambridge University Press, Cambridge, 2005CrossRefGoogle Scholar (ICRC Customary Law Study), Rule 49. See also DoD Manual, above note 46, § 5.17.3; UK Ministry of Defence, The Manual of the Law of Armed Conflict, 2004, para. 11.88.

65 San Remo Manual, above note 54, para. 108.

66 Ibid., para. 118. The term “merchant ship” (or vessel) is broadly interpreted. For instance, it is defined by the United Kingdom as “a vessel that is not a warship, an auxiliary vessel, or other state vessel (such as a customs or police vessel) and that is engaged in commercial or private service”. UK Ministry of Defence, above note 64, para. 13.5(k). See also San Remo Manual, above note 54, para. 13(i).

67 As set out in the San Remo Manual, “[a] blockade may be enforced and maintained by a combination of methods and means of warfare provided this combination does not result in acts inconsistent with the rules set out in this document”. Ibid., para. 97. While this paragraph acknowledges the potential use of e.g. mines, the effective enforcement of a blockade generally requires, in practice, the exercise of the right of capture, which is reserved to warships. Ibid., paras 98, 118, 146(f).

68 UNCLOS, Arts 39(1)(c), 53(3).

69 Ibid., Arts 39(1)(b), 54.

70 Ibid., Art. 19(2)(f).

71 Ibid., Art. 19(2)(l).

72 Pictet, Jean (ed.), Commentary to the Geneva Conventions of 12 August 1949, Vol. 1: Geneva Convention for the Amelioration of the Condition of the Wounded and Sick in the Armed Forces in the Field, ICRC, Geneva, 1960Google Scholar (ICRC Commentary on GC I), p. 32. See also Pictet, Jean (ed.), Commentary to the Geneva Conventions of 12 August 1949, Vol. 2: Geneva Convention for the Amelioration of the Condition of the Wounded, Sick, and Shipwrecked Members of Armed Forces at Sea, ICRC, Geneva, 1960 (ICRC Commentary on GC II), p. 28Google Scholar; Pictet, Jean (ed.), Commentary to the Geneva Conventions of 12 August 1949, Vol. 3: Geneva Convention relative to the Treatment of Prisoners of War, ICRC, Geneva, 1960, p. 23Google Scholar; Pictet, Jean (ed.), Commentary to the Geneva Conventions of 12 August 1949, Vol. 4: Geneva Convention relative to the Protection of Civilian Persons in Time of War, ICRC, Geneva, 1958, p. 20Google Scholar. This point was reiterated in both of the ICRC's updated commentaries published to date: ICRC Commentary on GC I, 2nd ed., 2016, § 236; ICRC Commentary on GC II, 2nd ed., 2017, § 258.

73 In international humanitarian law, an attack is defined as an “[act] of violence against the adversary, whether in offence or in defence”. Protocol Additional (I) to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts, 1125 UNTS 3, 8 June 1977 (AP I), Art. 49(1). While the treaty definition applies only to attacks from the sea that are directed at the land or may affect civilians on land, it is widely accepted as applying to naval warfare, generally, as custom. San Remo Manual, above note 54, para. 13(b).

74 DoD Manual, above note 46, § 6.2. See also Program on Humanitarian Policy and Conflict Research, Commentary on the HPCR Manual on International Law Applicable to Air and Missile Warfare, 2010, Rule 9, para. 2; Michael N. Schmitt (ed.), Tallinn Manual on the International Law Applicable to Cyber Warfare, 2013, Rule 48, para. 2.

75 ICRC, A Guide to the Legal Review of New Weapons, Means and Methods of Warfare, November 2006, § 1.2.

76 AP I, Art. 51(4)(b)–(c). On the customary law prohibition, see ICRC Customary Law Study, above note 64, Rule 71.

77 The principle of distinction requires that parties to a conflict “shall at all times distinguish between the civilian population and combatants and between civilian objects and military objectives and accordingly … direct their operations only against military objectives”: AP I, Art. 48. On the customary law status of the principle, see ICRC Customary Law Study, above note 64, Rule 1. As to the maritime context, see San Remo Manual, above note 54, para. 39.

78 Convention No. VIII relative to the Laying of Automatic Submarine Contact Mines, 36 Stat. 2332, TS No. 541, The Hague, 18 October 1907 (Hague Convention VIII), Art. 1(3). See also San Remo Manual, above note 54, para. 79.

79 Hague Convention VIII, Art. 1(1). See also San Remo Manual, above note 54, para. 82(b).

80 AP I, Art. 49(3).

81 San Remo Manual, above note 54, paras 40, 42(b)(i), 46.

82 Ibid., para. 42(b)(i). See also ICRC Customary Law Study, above note 64, Rules 11–12; AP I, Art. 51(4). Protected ships may sometimes lose their protection if they fail to meet the associated requirements. San Remo Manual, above note 54, paras 48–49; Geneva Convention (II) for the Amelioration of the Condition of the Wounded, Sick, and Shipwrecked Members of Armed Forces at Sea of 12 August 1949, 6 UST 3217, 75 UNTS 85 (GC II), Art. 34 (providing specifically for the loss of protection by hospital ships).

83 San Remo Manual, above note 54, paras 47. See also GC II, Arts 21, 22, 27, 38 (providing for protection of hospital ships and medical transports); Convention for the Protection of Cultural Property in the Event of Armed Conflict, 249 UNTS 240, 14 May 1954, Art. 12(3) (providing for the protection of vessels carrying certain cultural property); Convention No. XI Relative to Certain Restrictions with Regard to the Exercise of the Right of Capture in Naval War, 36 Stat. 2396, TS No. 544, 18 Oct 1907, Arts 3, 4 (providing for the protection of small coastal fishing vessels and small boats engaged in local coastal trade, and of vessels charged with religious, scientific or philanthropic missions).

84 San Remo Manual, above note 54, para. 60.

85 Ibid., para. 60(e).

86 Ibid., para. 136; GC II, Arts 21, 22, 27, 38 (providing for protection of hospital ships and medical transports); AP I, Art. 70 (providing for the protection of relief consignments).

87 San Remo Manual, above note 54, para. 135; US Commander's Handbook, above note 61, para. 8.6.2.1. See also James Kraska, “Prize Law”, in Max Plank Encyclopaedia of International Law, December 2009.

88 San Remo Manual, above note 54, para. 118.

89 Ibid., paras 114, 146.

90 Ibid., para. 41.

91 AP I, Art. 41(2)(b); San Remo Manual, above note 54, para. 47(i).

92 See, e.g., Sparrow, Robert, “Twenty Seconds to Comply: Autonomous Weapon Systems and the Recognition of Surrender”, International Law Studies, Vol. 91, 2015, pp. 703–712Google Scholar.

93 San Remo Manual, above note 54, para. 47.56–47.57.

94 DoD Manual, above note 46, § 5.10.3.3. See also ICRC Customary Law Study, above note 64, pp. 168–169.

95 San Remo Manual, above note 54, para. 46(d).

96 See discussion of this issue in Schmitt, Michael N. and Thurnher, Jeffrey C., “‘Out of the Loop’: Autonomous Weapon Systems and the Law of Armed Conflict”, Harvard National Security Journal, Vol. 4, 2013, pp. 253–257Google Scholar.

97 AP I, Art. 57(4).

98 Sandoz, Yves, Swinarski, Christophe and Zimmermann, Bruno (eds), Commentary on the Additional Protocols, ICRC, Geneva, 1987Google Scholar, paras 2230–2233.

99 San Remo Manual, above note 54, para. 46; DoD Manual, above note 46, §§ 5.11, 13.3 (“In general, the rules for conducting attacks, such as bombardments, by naval forces are the same as those for land or air forces.”); German Navy, above note 60, pp. 165–166.

100 For instance, see San Remo Manual, above note 54, para. 58. See also ICRC Customary Law Study, above note 64, pp. 35–36; but see DoD Manual, above note 46, § 5.4.3.2.

101 San Remo Manual, above note 54, para. 108.

102 Although its application in some circumstances has undoubtedly been modified by the Charter of the United Nations, the law of neutrality remains valid and relevant today. See, e.g., Bothe, Michael, “The Law of Neutrality”, in The Handbook of International Humanitarian Law, 3rd ed., 2013, pp. 552–554Google Scholar.

103 San Remo Manual, above note 54, para. 14.

104 Ibid., paras 23–30.

105 Ibid., para. 29.

106 Convention No. XIII Concerning the Rights and Duties of Neutral Powers in Naval War, 36 Stat. 2415, TS No. 545, The Hague, 18 October 1907 (Hague Convention XIII), Arts 9–10.

107 Hague Convention XIII, Art. 9.

108 San Remo Manual, above note 54, para. 16. See also Hague Convention XIII, Art. 2.

109 San Remo Manual, above note 54, para. 15.

110 Ibid., para. 30.

111 Ibid., paras 15, 22. These obligations are drawn, in part, from Hague Convention XIII, Art. 25. See also DoD Manual, above note 46, § 15.3.2.

112 San Remo Manual, above note 54, para. 22; DoD Manual, above note 46, § 15.4.2.

113 San Remo Manual, above note 54, para. 34.

114 Ibid., paras 36–37.