Attacking military aid before it gets to Ukraine
Your question mentions the concept of overt war which I think is a central concept in escalation. I think we can agree that there is at this point an overt war between Russia and Ukraine. Both of those parties are supported by other countries, for example through the supply of weapons, training or intelligence.
There is no overt war between the supplying countries and the warring countries, e.g. the United States is not directly at war with Russia even though it is providing military aid to Ukraine.
Legality from an international law perspective
I think different legal arguments can be made.
Russia's legal reasoning
Russia might reason that its war on Ukraine is justified and some Western countries have joined Ukraine's side in that war. In a way, Russia's Deputy Chair of the Russian Security Council has claimed that in December of 2022.
This reasoning works on the basis that Russia and Western allies supplying Ukraine with heavy weapons are already at war with Russia. Then, some of those supplies (barring exceptions) would be fair game from an international law perspective because they are military targets. For example, Article 52(2) of the 1977 Additional Protocol I to the Geneva conventions states:
- Attacks shall be limited strictly to military objectives. In so far as objects are concerned, military objectives are limited to those objects
which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a
definite military advantage.
Specifically, if the US provides Ukraine with M1 Abram tanks then they would in all likelihood be shipped to a port in Western Europe after which they would be transported east by train Here's a news report of such a transport.
Note that the clarification from Protocol I (the Geneva Conventions doesn't further define the concept of 'military objectives') does not distinguish between who controls the military objective. Even though tanks destined for Ukraine are transported across the Atlantic by US ships, they are still military objectives for Russia because they are directly linked to the war in Ukraine. More precisely, those ships fit the definition laid out in Article 52(2).
This treaty is relevant in this reasoning because Russia will argue that it is already at war with those Western allies supplying these weapons.
Legal reasoning why Russia would not be justified to attack
The Western allies supplying Ukraine would use a different reasoning. They would state that Russia's war of aggression on Ukraine is illegal. They help Ukraine through the principle of collective self defense which is enshrined in Article 51 of the UN Charter.
Participating in this self defense does not give Russia a legal right to self-defend against this collective self defense. As reasoned by Alexander Wentker in EJIL: Talk!:
At the outset, it should be recalled that whether a State has become a party to an international armed conflict has no bearing on the lawfulness of the use of force by or against the respective States. That question is governed solely by the jus ad bellum. Western States’ becoming party to the conflict alongside Ukraine would not entitle Russia to use force against them, since, even once States are parties to an armed conflict, every instance of use of force by and against them must be assessed against the prohibition of the use force. As others have shown, other States can lawfully assist Ukraine in collective self-defence against Russia’s armed attack (so long as they act in accordance with necessity and proportionality) and Russia has no claim to self-defence against this self-defence.
Pros and cons of Russian attacks on weapon supplies before they reach Ukraine
Taking out such a transport would be a moral victory for the Russians, significantly reduce Ukraine's war effort (at least for some time), and further stir up debate in the West whether supplying weapons isn't a greater risk than they are willing to take (the attack hits even closer to home).
This isn't without risk for Russia either though, because it amounts to attacking NATO forces in NATO's article 5 territory. Nevertheless, it's no certainty that an attack would lead to a NATO response.
For example, if the attack focuses on a US supply then the US might decide not to trigger article 5 consultations because they don't want further escalation through NATO. Instead, the US could do some proportional retaliation aimed at not escalating further. In that case, there is no overt war between the US and Russia but the level of escalations back and forth does go up.
Perspectives on attacking weapon supplies before they reach the conflict
From Russia's perspective, the attack might be seen as legitimate because the rhetoric has already shifted to being at war with NATO (rather than just Ukraine). Early on in the war Russia has also mentioned willingness to attack military aid. According to the New York Times:
Russia’s foreign minister, Sergey V. Lavrov, seemed to threaten such an attack, recently telling the state-run news outlet RT that “any cargo moving into Ukrainian territory which we would believe is carrying weapons would be fair game.”
Even from an outsider's perspective, Russian attacks on those US weapon supplies to Ukraine are as legal as Ukrainian attacks on Russian military convoys dedicated to the war in Ukraine but still physically located in Russia. The latter has been widely acknowledged in the West as a legal course of action on Ukraine's part.
Historic precedence
A New York Times article dating back to March of 2022 argued it's unlikely that Russia would attack Western supply lines based on historic precedence.
Since then, I think the conflict has developed a lot. Back then the West supplied simple weapons while this has now escalated to the supply of main battle tanks. Nevertheless, one of the article's main points still holds:
The prospect of direct conflict with the nuclear-armed NATO alliance provides President Vladimir V. Putin of Russia with a powerful disincentive.
On the other hand, most of the steps in the escalation ladder have since been taken. If Russia does want to escalate proportionally (i.e. without the back and forth of escalations spiralling out of control) then this seems like one of the options.