Prima facie, it is Russia which is acting illegally in the Kerch Strait. As I wrote when it was the Russians who were being harassed in the English Channel:

Contrary to Article 44 of the UN Convention on the Law of the Sea, to which the UK and Russia are both party, the UK has engaged in extensive illegal harassment of a Russian naval submarine engaged in fully lawful transit of the Dover Strait. A Russian naval vessel en route between the Baltic and Black Seas is fully and specifically entitled under the UN Convention on the Law of the Sea Articles 37 and 38 to the right of passage through the strait. This is in addition to the general right of passage through the territorial sea at Article 17. The Russian navy was in full compliance with the provision at Article 20 that, while in territorial waters, the submarine must be on the surface and displaying its flag, and in compliance with Articles 29 to 32 on warships. Not only does the Russian Navy have every right to sail through the Dover strait on passage, it has been exercising that right – along with many other navies – for over a hundred years. The decision of the British government now to employ military harassment and threat is not only illegal, it is a gross and entirely deliberate act of provocation designed to sour international relations and disturb the atmosphere of world peace. The author of this article, Craig Murray is a former Head of the Maritime Section of the United Kingdom Foreign and Commonwealth Office, and former Alternate Head of the United Kingdom Delegation to the United Nations Preparatory Commission on the UN Convention on the Law of the Sea. He is a retired British Ambassador.

Russia is very definitely acting illegally in putting military personnel of another state on television to make statements, whether coerced or not (personally I found them precisely as believable – no more and no less – as Yulia Skripal’s strained statement to British TV).

Please note that Ukrainian ships have the right of innocent passage through the Kerch Strait irrespective of whether the Crimean side is viewed as Ukrainian or Russian. The coastal state does have the right to make arrangements for maritime safety which may include designating sea lanes and a notification regime akin to air traffic control. If Ukraine violated these provisions, (which seems probable), Russia had a right to take enforcement action. But that enforcement action specifically does not extend to substantive detention of vessels and crew.

The situation changes if Russia genuinely has evidence that the military vessels were engaged in a military attack. But it only changes, and the civilian rules only cease to apply, if one side or the other acknowledges that a state of war now exits. Ukraine came close to this

by demanding that its servicemen be treated as prisoners of war. There is no option to treat uniformed military personnel of another state as terrorists. But if Russia does not acknowledge a state of war, it has to let them go. Russia is certainly not entitled to impose a wider blockade of the strait to shipping to or from Ukraine – any more than Israel is entitled to blockade Gaza.

Given that Russia appears on the face of it to be very much in the wrong, the western powers have been remarkably quiet. I suspect this indicates knowledge that Poroshenko was indeed engaged in some sort of stupid stunt. In which case the Russians have played into his hands by a disproportionate reaction. Poroshenko’s own action in declaring martial law is of course also wildly disproportionate. My sense is that we have here two Presidents each with slipping popularity ratings, deliberately escalating a crisis as it suits each domestically. Such playing with fire is wildly irresponsible, far too many people have died in Ukraine already.

I expect the usual howls of protest from people for whom the application of impartial international law is anathema, who believe you must be on the side of the “goodies” against the “baddies”. I am aware that rationality and impartiality are not much valued in political discourse nowadays. I shall however stick to them with stoic resolve.