It's a casus belli.
First up, referring back to February 6ths "The RAND Corporation Blueprint For Forcing Putin To Over-Extend Himself"
I hope that the U.S. or NATO or whoever commissioned this study
didn't pay a lot for it, it's basically the strategy that Pope John Paul
II, Margaret Thatcher and Ronald Reagan came up with in the early
1980's although the details do differ. The tactical components of the
RAND plan are:
1. Arming Ukraine ;
2. Increase support for jihadists in Syria;
3. Promoting regime change in Belarus;
4. Exploiting tensions in the South Caucasus;
5. Reducing Russian influence in Central Asia;
6. Rivaling the Russian presence in Transnistria.
....MUCH MORE
On January 12 Victoria Nuland showed this approach is top-of-mind in the Biden Administration. From Interfax Ukraine:
Nuland: I'm going to let Russians speak for themselves how long they can financially back placement of troops near Ukraine
And from BNE IntelliNews, April 26:
Russian
media have been reporting claims by figures including a former minister
from Ukraine’s separatist Donetsk region that EU and Nato member
Romania is preparing to take Moldova’s pro-Russian Transnistria region
by force.
The bizarre claims were widely reported in the Russian media
following the first of a series of as yet unexplained attacks on targets
within Transnistria, a Russia-backed separatist region in the east of
Moldova, bordering Ukraine. There is no evidence to support them, yet
they raise some worrying questions about Russia's future intentions
towards Transnistria.
As reported by bne IntelliNews, after news broke of an attack from a grenade launcher later identified
as a RPG-27 Tavolga on the empty headquarters of the Ministry of State
Security (MGB) in Tiraspol, the Russian public were inundated with
propaganda claiming that Romania has nearly invaded Moldova and is
preparing to oppress Russian-speaking nationals in Transnistria.
“Romania, with the support of Nato and with the participation of the
Ukrainian army, plans to seize Transnistria and carry out massive
political repressions against all supporters of Russia,” Russian daily
Pravda announced immediately after the incident in Tiraspol, quoting politologist Sergei Markov.
The so-called expert claimed that the Romanian army has already been deployed in Moldova and forces are being built up.
Earlier, the former minister of defence of the unrecognised Donetsk People's Republic (DPR) in eastern Ukraine, Igor Strelkov, claimed that Romania was conducting covert preparations to capture Moldova.
According to him, a significant part of the Moldovan officers in all
headquarters in all key positions have been replaced by officers of the
Romanian army.
The unfounded claims have not yet received any official response from
Romania, a staunch backer of Ukraine in the war against Russia, as well
as a supporter of Moldova, whose economy has been hard hit by the war
on its eastern border.
While they are unlikely to carry any weight in either Moldova or
Transnistria, set against observations on the ground, they may help
convince some of the Russian public should Moscow be gearing up for
action in Moldova.
Moldova is seen as the most vulnerable country to a potential
spillover from the conflict, given its status as a small post-Soviet
state, close to the fighting and with part of its territory occupied by
Russia-backed separatists.
Officials in both Chisinau and Tiraspol have so far largely refrained
from comments that could lead to an escalation of the situation. While
condemning the invasion, the Moldovan government has insisted on
maintaining its neutrality and unlike Ukraine, has not been pursuing
Nato membership, although it recently applied to join the EU.
For its part, the government of Transnistria has linked the attacks
to Ukraine. The unrecognised republic's President Vadim Krasnoselsky
commented on April 26: "I assume that those who organised this attack
have the goal of dragging Pridnestrovie into the conflict."
Tensions between Chisinau and Moscow increase
However, tensions between Chisinau and Moscow have increased in the
last week, after Russian Brigadier General Rustam Minnekayev said on
April 22 that the Russian army plans to take Ukraine's Donbas region,
then create land corridors to both the Crimea and Transnistria.
"Since the beginning of the second phase of the special operation,
which has already begun, literally two days ago, one of the tasks of the
Russian army is to establish full control over the Donbas and southern
Ukraine," said Minnekayev, the acting commander of Russia’s Central
Military District, as reported by Tass.
"Control over the south of Ukraine is another way out to
Pridnestrovie [Transnistria], where there are also facts of oppression
of the Russian-speaking population. Apparently, we are now at war with
the whole world, as it was in the Great Patriotic War, all of Europe,
the whole world was against us. And now the same thing, they never liked
Russia," added Minnekayev....
....MUCH MORE
If the Russians take Odessa the resulting Ukraine would be a landlocked rump state similar to the Russophile/Europhile Presidential election map from a dozen years ago:
Daily Mail, February 23, 2014
Or, going a bit further back in time, as noted February 28:
....And if I didn't know better—seriously, who would be that
revanchist—I'd say the Russians are trying to recreate the Novorossiya
(New Russia) of Catherine the Great:

We didn't have all those posts on Crimea for nothing you know.....