This makes sense only if they are transported by Ukrainian vessels/trains/planes etc. If the USA is shipping military hardware in a military transport plane, vehicle or vessel, attacking the shipment would entail also attacking the USA. I don't think the USA calls an Uber to get their stuff to where it is going.

@PoloHoleSet It does entail also attacking the USA, but it would still be justified if it is clear that a primary purpose of that ship is supplying weapons to Ukraine. Whether the tanks are transported by the US or Ukrainian military doesn't really matter, they are a legitimate military target. That is, attacking them would not be an unprovoked attack or a war crime.

Attacking military aid before it got to Ukraine would likely be seen as an act of war by NATO or the countries sending it.

@JoeW yes, that's the risk. At this point there isn't much room for escalation short of what can be seen as acts of war, I think. Nevertheless, I think there's a good chance that it would not lead to a full-blown war. Instead, the US would probably try to retaliate in a way that would not evoke much further retaliation from Russia while evening the score for this attack. Of course that's hard, because any proportionate reaction would also be seen by Russia as an act of war. Russia knows the US would have to retaliate, so hopefully this deterrence is enough for it to not escalate in this way.

I understand that is a risk but I think that qualifies as an overt act of war that the question was looking to exclude.

@JJJ Hard to imagine Russia thinking it could attack US transportation w/o starting a general war with NATO. The most I could imagine would be attacking supply depots neighboring Ukraine. Which would probably cause the USA to commit to seriously defending Ukrainian and neighboring airspace. Something that would interfere with Russia's terror campaign against civilian targets.

@JoeW right, the way I read it initially was that it shouldn't lead to an overt war. I based that mostly on reading the title question. Technically speaking, an act of war is something that provokes a war. Since I'm arguing here that it may not actually start a war (instead being confined to a limited exchange) I think it's okay to keep the answer up.

@DaveG I think there's a reasonable chance that Russia could attack such a supply without it ending in a full-blown war. The reason for that is that the US doesn't want to be involved in such a war. I think Russia doesn't either. So it's in both of their interests to limited any escalation to an exchange of proportionate responses. I don't think the US can commit to defending Ukrainian airspace, that would make them a party to the current war because they would be directly involved in shooting down Russian aircraft. This discussion has been had already if you search for the term 'no fly zone'.

@DaveG I think it's a very complicated situation. The US has held the position that it's not at war with Russia while Russia is at or close to the position that NATO is waging war against Russia. So for Russia it's a small leap to attack a US vessel with weapons destined for the war in Ukraine because it's a valid military objective and it fits their narrative. The US doesn't want to be directly involved in a fighting role and maintains the position that they're just helping Ukraine. So while legally the two are similar, I think it's easier for Russia to escalate in this way.

@DaveG As for your other points. At this time, no escalation is without or even with low risk because the war has already seen so much escalation. As for shooting down missiles and drones, the US and others are already helping with that by supplying weapons, but not enough to defend the whole of Ukraine. Even with all possible help, it's probably not possible to intercept every missile (and drone) if you consider the size of Ukraine. And it's also not super efficient. The Iranian drones Russia uses cost some $25k, many of the anti-air ammunition is more expensive.

Anyway, much of this is kind of getting off-topic. For general discussion about the war, please go to the Ukrainian Conflict 2022 chat room, I'm happy to elaborate on some of these points in more detail over there. If someone has more criticism of my answer here or some suggestions for improvements, that's okay to post here of course. :)

This isn't without risk for Russia either though, because it amounts to attacking NATO forces in NATO's article 5 territory — attacking a ship in the middle of the Atlantic would not be covered by NATO Article 5, because that only covers attacks in Europe or North America?

Attacking supply ships may be legal, but historic precedent (WW1, WW2) show that it may still to lead to significant responses and publicity shifts.

@lidar yes, I agree that it woud be a big thing with significant responses and public opinion shifts as you expect. Regarding the latter, I wonder how those actually turn out. Rather than providing a united front against Russia, I would also expect for it to deepen public opinion and political divisions on whether the West should actually be involved.

You compare Russia attacking NATO supply routes with Ukraine attacking Russian supply routes. I think this is in error: Ukraine and Russia are in a state of war. Russia might call it "terrorism" when Ukraine attacks Russian territory, but as far as international law is concerned, such is clearly fair game. Russian bombers attacking for example Dutch or German harbors, or Polish railroads, would clearly be an attack on the territory of a non-combattant nation though. Sweden sold / delivered iron to Nazi Germany, and was not attacked / still considered neutral.

@lidar: indeed, the US declaration of war in 1917 kinda leads with the unrestricted submarine warfare en.wikipedia.org/wiki/… Nowadays the US would probably be much more skittish since Russia has nukes, but this could easily escalate in an undeclared global naval conflict, i.e. US navy seizing all ships bound for Russia, Russia escorting (some) of their ships with their navy which could lead to direct naval confrontations etc. It would be more feasible if Russia did this Iranian-style in semi-deniable fashion, but that would limit scale.

@DevSolar no, that's my point. Even with the port or transport being conducted by a third country, it is still a military objective because it meets the description laid out in Article 52(2) mentioned in my answer. Note that the countries you mention are not neutral, they are actively helping Ukraine even if they are not directly participating in the war. Because the weapons are destined for the war in Ukraine, there is sufficient military necessity to target such shipments and it can be done without harming civilians disproportionately.

The article you are citing is about "Parties to a Conflict". It is highly questionable if selling (or, indeed, gifting) arms to a nation in conflict is making a third party "party to the conflict". If you are argueing this way, basically every country at war since WWII would be justified to attack either Russian or American harbors and shipping, because either US or USSR weapons were supplied to the opposition.

@DevSolar I see the Russians have also made this argument last December, I'll add that into my answer later today. I am not sure I agree with this argument to the extent that all of NATO is a legitimate military target, but I do think the reasoning holds for those weapons destined for Ukraine. If you don't agree with my reasoning then it seems this would be a weird loophole in international law where you can claim neutrality on the one hand while actively helping an ally through the supply of advanced weapons and intelligence.

@DevSolar we could also go back a little more to the 1907 Hague Convention IX, article two starts off with "Military works, military or naval establishments, depots of arms or war ' matériel, ' workshops or plant which could be utilized for the needs of the hostile fleet or army, and the ships of war in the harbour, are not, however, included in this prohibition." The wording used here, "utilized for the needs of the hostile fleet or army" would to my mind apply to the example in my answer too. Even if there is no clear precedent, I'd argue the reasoning is sound.

Note that I was very deliberately not talking about supply ships in international waters, for which a point could be made. Attacking ships in harbor, trains etc. does involve violating that country's territory / airspace, though -- the weapons in that train might be bound for Ukraine, so the train might be a valid target, but attacking it while in another country definitely constitutes an attack on that country.

Again, that quote is from a treaty addressing parties to a conflict. That's circular reasoning: You claim that such supplies are valid targets because that country is party to a conflict because such supplies are valid targets.

@DevSolar Yes I do get your reasoning saying that selling weapons to a country at war doesn't necessarily involve the selling country in that war. I also agree that if the attack takes place in a port or on their soil then it would also be an attack on that country. I guess it's one of those cases where you can present different legal arguments to justify the conclusion you want. For Russia, that might involve arguing they are already at war with NATO countries because they don't just supply weapons, they are more actively involved (e.g. by providing intelligence / target info as well).

@DevSolar after reading some more I think the whole point about whether the West is a party to the conflict might be moot. The West is helping Ukraine defend its territory on the basis of collective self-defense. Based on that situation Russia does not have a legal right to attack those collective self defense countries in the same way that they weren't justified to attack Ukraine in the first place. See here if you're interested in the legal argument.

@JoeW NATO would see it as an act of war; Russia would see supplying the tanks as an act of war, and its own actions as a justified prevention of NATO's invasion of Russia.

@user253751 And the question was about what Russia could do that wouldn't be seen as an overt act of war. It wasn't asking how Russia saw the act of providing tanks as aid. That said I don't think it would just be NATO that saw an attack on aid before it got to Ukraine as an act of war.

@JJJ - I did kind of get that from your answer, which is why I asked for clarification. The problem is the question didn't ask what they could do that might be considered justified, or righteous, the question asked, "short of overt war" - and that, absolutely, would be overt war.