Mariupol has become the most heavily bombed and damaged city in Ukraine's war with Russia - having suffered the brunt of sustained Russian attacks. It is key to Moscow's military campaign in Ukraine. But why?
There are four main reasons why taking the port city would be such a strategic win for Russia - and a major blow for Ukraine.
1. Securing a land corridor between Crimea and Donbas
Geographically, the city of Mariupol occupies only a tiny area on the map but it now stands obstinately in the way of Russian forces who have burst out of the Crimean peninsula.
They are pushing north-east to try to link up with their comrades and Ukrainian-separatist allies in the Donbas region of eastern Ukraine.
General Sir Richard Barrons - former commander of UK Joint Forces Command - says capturing Mariupol is vital to Russia's war effort.
"When the Russians feel they have successfully concluded that battle, they will have completed a land bridge from Russia to Crimea and they will see this as a major strategic success."
If Mariupol was seized, Russia would also end up with full control of more than 80% of Ukraine's Black Sea coastline - cutting-off its maritime trade and further isolating it from the world.
By holding out against advancing forces for the past three weeks, the defending Ukrainians have managed to preoccupy a large number of Russian troops. But that failure by Russia to secure a rapid capture of the city, has prompted Russian commanders to resort to a 21st Century version of mediaeval siege tactics.
They have pummelled Mariupol with artillery, rockets and missiles - damaging or destroying over 90% of the city. They have also cut off access to electricity, heating, fresh water, food and medical supplies - creating a man-made humanitarian catastrophe which Moscow now blames on Ukraine for refusing to surrender by an 05:00 deadline on Monday. A Ukrainian MP has accused Russia of "trying to starve Mariupol into surrender".
Ukraine has vowed to defend the city down to the last soldier. It may well come to that. Russian troops are slowly pushing into the centre and, in the absence of any kind of workable peace deal, Russia is now likely to intensify its bombardment - drawing little if any distinction between its armed defenders and the beleaguered civilian population which still numbers over 200,000.
If, and when, Russia takes full control of Mariupol this will free up close to 6,000 of its troops - organised into 1,000-strong battalion tactical groups - to then go and reinforce other Russian fronts around Ukraine.
There are a number of possibilities as to where they could be redeployed:
- to the north-east to join the battle to encircle and destroy Ukraine's regular armed forces fighting pro-Kremlin separatists in the Donbas region
- to the west to push towards Odesa, which would be Ukraine's last remaining major outlet to the Black Sea
- to the north-west towards the city of Dnipro
2. Strangling Ukraine's economy
Mariupol has long-been a strategically important port on the Sea of Azov, part of the Black Sea.
With its deep berths, it is the biggest port in the Azov Sea region and home to a major iron and steel works. In normal times, Mariupol is a key export hub for Ukraine's steel, coal and corn going to customers in the Middle East and beyond.
For eight years now, since Moscow's illegal annexation of Crimea in 2014, the city has been sandwiched uncomfortably between Russian forces on that peninsula and the pro-Kremlin separatists in the breakaway self-declared republics of Donetsk and Luhansk.
Losing Mariupol would be a major blow to what is left of Ukraine's economy.
3. Propaganda opportunity
Mariupol is home to a Ukrainian militia unit called the Azov Brigade, named after the Sea of Azov which links Mariupol to the rest of the Black Sea. The Azov Brigade contains far-right extremists, historically including neo-Nazis.
Although they form only the tiniest fraction of Ukraine's fighting forces, this has been a useful propaganda tool for Moscow, giving it a pretext for telling Russia's population that the young men it has sent to fight in Ukraine are there to rid their neighbour of neo-Nazis.
If Russia manages to capture alive significant numbers of Azov Brigade fighters it is likely they will be paraded on Russian state-controlled media as part of the ongoing information war to discredit Ukraine and its government.
4. Major morale boost
The capture of Mariupol by Russia, if it happens, will be psychologically significant for both sides in this war.
A Russian victory in Mariupol would enable the Kremlin to show its population - through state-controlled media - that Russia was achieving its aims and making progress.
For President Putin, for whom this war appears to be personal, there is a historical significance to all this. He sees Ukraine's Black Sea coastline as belonging to something called Novorossiya (New Russia) - Russian lands that date back to the 18th Century empire.
Putin wants to revive that concept, "rescuing Russians from the tyranny of a pro-western government in Kyiv" as he sees it. Mariupol currently stands in the way of him achieving that aim.
But to Ukrainians, the loss of Mariupol would be a major blow - not just militarily and economically - but also to the minds of the men and women fighting on the ground, defending their country. Mariupol would be the first major city to fall to the Russians after Kherson, a strategically much less important city that was barely defended.
There is another morale aspect here and that is of deterrence.
Mariupol has put up fierce resistance - but look at the cost. The city is decimated, it lies largely in ruins. It will go down in history alongside Grozny and Aleppo, places that Russia eventually bombed and shelled into submission, reducing them to rubble. The message to other Ukrainian cities is stark - if you choose to resist like Mariupol did then you can expect the same fate.
"The Russians couldn't walk into Mariupol," says Gen Sir Richard Barrons, "they couldn't drive in with their tanks, so they've pounded it to rubble. And that's what we should expect to see anywhere else that really matters to them."
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