The only "rules" that exist in a war are what are mutually beneficial to both sides.
The corpus of law of armed conflict exists for everyone; while its codification is relatively recent(late 19th century onwards), as a customary law it existed pretty much since the beginning of war.
Specific rules on commerce warfare were invented on behalf of leading maritime nations and neutral nations.
Ukraine can undermine them - sure. It can fight on its own, too, because those aren't rules US/UK would really like to see undermined. And the risks here are larger than the risk of Russia winning.
Yes, Ukraine can play an unrestricted interdiction campaign(in fact it just tested the waters) with a weapon system that specifically
can't do a legal blockade. So can Russia - and just by blocking loading facilities to riverine ports (which is perfectly doable, and now technically can be done in reciprocation) will leave Ukraine limited to land transport only.
Also, just to make sure - simply because Ukraine can escalate no further than the Black Sea - doesn't really mean Russia can't either.
The moment you begin a war, you automatically break all rules and enter the phase where violence reigns supreme.
People who thought this way were quite famously hanged(or shot themselves in a bunker). Mostly on the losing side, sure, but some of the winners, too.
There are no strict rules in warfare.
There is a whole body of laws regulating both entry (jus ad bellum) and the armed conflict itself (jus in bello). It's quite developed, has lots and lots of commentary/practice, and is very well established.
While there were lots of separate instances of war crimes on both sides(there are crimes against all laws), it's worth pointing out that generally, both sides tend to stick to the laws of war. Yes, Ukraine is generally more willing to test the boundaries. The risk to overdo it is also one of Ukrainian main risks in the war, because it is fighting on external life support.
Sure if it doesn't want to get nuked back or (most likely) conventionally annihilated by NATO.
Nuked back by Ukraine? It doesn't have means or weapons. Nor will be provided with them, because with nuclear weapons, it's the original owners' fault.
Nuked by NATO? That's a game for two, and while an all-out war will very technically favor the general West (conventional superiority of survivors), at that point it won't matter. To be fair, I wonder if a "united West" will even remain a thing when nukes will start flying around.
Declaring armed neutrality in this case is a very obvious answer.
Conventionally annihilated? Well, I don't remember anyone sane who thinks the Russia-NATO conflict will be conventional. Since the 1950s, nukes are the equalizer of the weaker side - and just so happens that Russian tactical nukes are more than an order of magnitude more plentiful than NATOs(leading to a significant overmatch in overall numbers and delivery means in Russian favor).
TLDR be careful what you wish for.
If Russia was powerful enough it could do whatever it wanted. That it cannot do so, its because its strength is lacking.
It already started a war, which no one interferes with.
Thats called deterrence. Ukraine can break the "rules" because Russia can't deter it with its own means because it is already deterred by NATO or can't impose high enough costs to it by conventional means
The deterred side here is NATO. It's the first conflict in a very long time where the West can
not interfere directly.
Ukraine can try to break the rules because it's desperate - and because it's Russian wish to limit the conflict without needless escalations - it plays a long game.
Russia doesn't use nuclear weapons not for the fear of Western retaliation(it ironically is a secondary concern), but (1)because it goes against the basic goals of the war, which is to get a
neutral Ukraine, (2)won't be supported inside, (3)will break the unspoken deal with the "global South", China and the rest of the world.
But if pushed enough - sure, you can get to the point where there will be flashes. And this point will probably involve all of us and our families very personally, so please behave.
