[jasode]

>, because Russia controls Sevastopol since it annexed Crimea in 2014; it didn't need the current war to control it more.

Crimea's fresh water supply came from a canal originating in Ukraine. After Russia's annexation, Ukraine retaliated by blocking the water flow with concrete so Crimea was running out of water. Deep link to explanation: https://youtu.be/If61baWF4GE?t=16m27s

So it was no surprise that on the 2nd day of the invasion, Russian soldiers used explosives to blow up Ukraine's dam to to re-open the canal: https://www.youtube.com/shorts/313jB_tLSxQ

Crimea residents' (many pro-Russian) reaction a week later on March 3: https://www.youtube.com/watch?v=jBJmnMqH8S4

Also, Russia controlling Ukraine's land and coastline from Donbass through Mariupol and down to north Crimea means they have a more viable "land bridge" for supplies than the tiny man-made bridge that links southern Crimea with Russia. Ukraine becoming allied with NATO (or a defacto "NATO ally" via EU membership instead of a Russia puppet) means that tiny bridge over the Kerch strait becomes a vulnerable chokepoint that threatens Russia. Deep link to that explanation: https://youtu.be/l5KXeFdpyaE?t=7m22s

From Russia's point of view, there are many desirable military objectives for them to take over southeastern Ukraine.

EDIT reply to: >, because a naive look at the map suggests that even without Crimea, Russia already has coastline onto the Black Sea between its borders with Ukraine and Georgia... there must be a strong reason in favour of invading another country rather than developing ports on the land they already have,

Russia's existing coastline with the Black Sea (e.g. Sochi, etc) has no deep water ports that connect to their major navigable rivers. The 2nd video I linked illustrates that. You can also see on the wikipedia diagram that the Russia's existing Black Sea coastline around Sochi doesn't connect to Russia's main river system: https://en.wikipedia.org/wiki/Unified_Deep_Water_System_of_E...

When Ukraine had pro-Russian president Viktor Yanukovych, the Crimea chokepoint wasn't as big of a threat. With the 2014 regime change to pro-NATO presidents Poroshenko & Zelenskyy, that mindset changed.

[FearNotDaniel]

I'm obviously missing some facts about the region, as per the OP's hypothesis, because a naive look at the map suggests that even without Crimea, Russia already has coastline onto the Black Sea between its borders with Ukraine and Georgia... there must be a strong reason in favour of invading another country rather than developing ports on the land they already have, if the goal is simply "control over the Black Sea and trade access to the Mediterranean Sea".

[bambax]

As per the video https://youtu.be/If61baWF4GE), above and beyond the geopolitical reasons for Russia's invasion of Ukraine, the main motive is economical:

1. Russia is in effect a petrostate; it lives and dies by its exports of natural gas and oil.

2. Immense fields of underwater natural gas and oil were discovered in 2012 in the Crimea maritime exclusive economic zone.

3. If Crimea stays with Ukraine, it lets Ukraine become a new petrostate and challenge Russia (take over its European customers), which to Russia is an existential threat. Hence the annexation of Crimea in 2014: not only for Sevastopol and access to the Mediterranean, but to exploit the oil fields (and/or prevent Ukraine from exploiting them).

4. But the supply of 80% of the fresh water to Crimea comes from a canal [0] that connects it to the Dnieper river in Ukraine; after the annexation, Ukraine shut the canal down, effectively thirsting Crimea to death.

5. If Russia wants to stay in Crimea (which it believes is necessary for its own survival), it needs to reopen that canal, and therefore control at least the southern part of Ukraine and possibly its whole Mediterranean coastline.

(It's not just about the canal, but the canal is a very big part of it.)

[0]: https://en.wikipedia.org/wiki/North_Crimean_Canal

[scioxo]

Russia's Black Sea coast is shallow and lacks large, sheltered harbours. It would cost billions and takes decades to build a proper port there.

Russia had actually be leasing the port at Sevastopol from the Ukraine since the fall of the USSR and had their Mediterranean fleet based there even before the invasion in 2014. They had a lot to lose in an anti-Russian Ukrainian government.

[bambax]

> Deep link to explanation: https://youtu.be/If61baWF4GE

This video is fascinating and explains a lot. Many thanks for this!!