Limits of navy jurisdiction/authority in international waters
<p>Once and then I read news about US navy stopping, searching and even sinking commercial vessels flowing non-US ensign (most recently Iranian).  Sometimes news regard navy vessels not conforming to COLREG and threatening other vessels with deadly force to give way instead.</p>
<p>I know that navy (US Navy especially) are guys with big guns and Iran is considered bad guys by many, but what are the (international) legal grounds of such conduct?</p>
<p>I know that some misconduct (like piracy) falls under universal jurisdiction, which enable any navy to intervene, but cases I have read about regarded good drug/fuel &quot;trafficking&quot;.  Is not a naval blockade an act of war?</p>
<p>For the sake of answering the question assume that:</p>
<ul>
<li>the state of ensign of the vessel is not a party of any international agreement that grants jurisdiction/authority to navy of any other state in case of the &quot;misconduct&quot;,</li>
<li>the &quot;misconduct&quot; is either legal in the state of ensign, or the state is not interested in its prosecution.</li>
</ul>

<p>The limit on boarding is contained in <a href="https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf" rel="nofollow noreferrer">Article 110</a> of the UN Convention on the Law of the Sea</p>
<blockquote>
<p>Article 110</p>
<p>Right of visit</p>
<ol>
<li>Except where acts of interference derive from powers conferred by
treaty, a warship which encounters on the high seas a foreign ship, other than
a ship entitled to complete immunity in accordance with articles 95 and 96,
is not justified in boarding it unless there is reasonable ground for suspecting
that:</li>
</ol>
<p>(a) the ship is engaged in piracy;</p>
<p>(b) the ship is engaged in the slave trade;</p>
<p>(c) the ship is engaged in unauthorized broadcasting and the flag
State of the warship has jurisdiction under article 109;</p>
<p>(d) the ship is without nationality; or</p>
<p>(e) though flying a foreign flag or refusing to show its flag, the ship
is, in reality, of the same nationality as the warship.</p>
<ol start="2">
<li><p>In the cases provided for in paragraph 1, the warship may proceed
to verify the ship's right to fly its flag. To this end, it may send a boat under
the command of an officer to the suspected ship. If suspicion remains after
the documents have been checked, it may proceed to a further examination on
board the ship, which must be carried out with all possible consideration.</p>
</li>
<li><p>If the suspicions prove to be unfounded, and provided that the ship
boarded has not committed any act justifying them, it shall be compensated
for any loss or damage that may have been sustained.</p>
</li>
<li><p>These provisions apply mutatis mutandis to military aircraft.</p>
</li>
<li><p>These provisions also apply to any other duly authorized ships or
aircraft clearly marked and identifiable as being on government service.</p>
</li>
</ol>
</blockquote>
<p>COLREG requires that all vessels avoid collisions; there is no <a href="https://www.usni.org/magazines/proceedings/2017/november/which-ship-has-right-way" rel="nofollow noreferrer">right-of-way</a> in navigation. A warship is generally more maneuverable than a merchant vessel and therefore has a greater ability (and responsibility) to avoid collisions. However, a warship executing a war-like function against a non-complying vessel also has more leeway if a collision occurs.</p>
<p>Of course, Article 95 of UNCLA gives a warship complete immunity other than in their home state for acts on the High Seas, so an Iranian merchant ship that wished to pursue damages would need to do so in the US Admiralty Courts.</p>

international jurisdiction military law-of-the-sea extraterritorial-jurisdiction