Mariupol is not among the places which became symbolic, or even particularly well known, to the outside world when the flames of separatism spread through the Donbass: but what happens there in the next days can be a game-changer in Ukraine's increasingly bloody and vicious civil war.
The forces of the Kiev government have made major gains in recent months, taking over the rebel stronghold of Slovyansk; surrounding the second largest city in the region, Luhansk; arriving at the gates of Donetsk, the capital of the 'Peoples' Republic'. But an unexpected counter-offensive has seen the separatists open a second-front, seize territory and, crucially, threaten to open a land corridor to Crimea giving the Kremlin full control of the Azov Sea.
Retreating Ukrainian forces are digging in around Mariupol, a port just 30 miles from Russia, to stop this breakthrough from taking place; a strategically important town nearby, Novoazovsk, is under prolonged attack, say Kiev, with columns of tanks from across the border forcing entry after prolonged artillery bombardment.
So, the focus on the ground will be on Mariupol. It is, perhaps apposite that the showdown will take place there; the city illustrated the enmity between bitter enemies. Government forces, backed by private armies of right-wing nationalists, bankrolled by oligarchs, carried out an attack there on one of the most revered days in Russian-speaking part of the country - the commemoration of victory over Nazi Germany. “This is not about 2014 in Ukraine, this is about Berlin in 1945, that is what they want to avenge, the defet of their Nazi masters”, Captain Zorin Aleksandr Nicolaivitch, a naval veteran of 18 years, told me in a voice of rage.
The Ukrainian president, Petro Poroshenko, who says he wants to heal the bitter divide in his country, has asked for an emergency session of the UN security council, while in Brussels Nato issued images showing “concrete examples of Russian activity inside Ukraine” and stated that there are around a thousand Russian soldiers now inside Ukraine.
But such are the changed dynamic that Moscow, while issuing routine official denials about the involvement of its forces, appeared relaxed about state TV acknowledging for the first time that serving military personnel had gone over to fight - although they had done so as volunteers sacrificing their beach holidays.
Strong indications of Russian presence in another country, an ally of the West which wants to join the EU and Nato, should lead to massive international outcry. But although David Cameron declared that he was “extremely concerned” about the provocative and unacceptable“ reports of incursion, Angela Merkel called Vladimir Putin to stress they ”must be explained“ and Francois Hollande declared that it would be ”intolerable and unacceptable“, there were no indications of immediate retaliatory action.
Ukraine crisis: Russian 'aid' convoy
Ukraine crisis: Russian 'aid' convoy
1/11 Ukraine crisis
Drivers of the first trucks of the Russian aid convoy parked in the city of Luhansk on 22 August
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An employee inspects the contents of a truck with Russian humanitarian aid in Mariupol, Ukraine on 22 August 2014
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The convoy nearing the border before it parked at a camp in Russia
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Russian convoy carrying humanitarian aid for residents in rebel eastern Ukrainian regions moving along a road in the city of Voronezh, about 530 km from Moscow, Russia
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An Ukrainian border guard checks passing cars at a checkpoint of Pletnyovka, Kharkiv region on Ukraine-Russia border, where Russian humanitarian convoy is to cross the border
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Ukrainian border guards stand at the Ukrainian-Russian border crossing
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Trucks of a Russian convoy carrying humanitarian aid for Ukraine are parked at the military air base outside Voronezh
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Drivers of a Russian convoy of trucks carrying humanitarian aid for Ukraine take a rest on a side of a road near the city of Yelets
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An Ukrainian soldier stands guard at a checkpoint of Pletnyovka, Kharkiv region on Ukraine-Russia border, where Russian humanitarian convoy is to cross the border
10/11 Ukraine crisis
A Russian convoy carrying humanitarian aid for residents in rebel eastern Ukrainian regions moves along a road about 50 km from Voronezh, Russia, 14 August 2014. The convoy continues to advance through Russian territory after a one-day stop in Voronezh in full coordination with and under the aegis of the Red Cross, according to Russian authorities
11/11 Ukraine crisis
The Russian convoy carrying humanitarian aid in the Voronezh region of Russia en route to Ukraine
Western powers had been distracted by the Gaza war and now over the bloody ascent of Isis. Trying, with some difficulty, to get together a coalition to bomb the Islamists in Syria, Barack Obama is not going to become engaged in a confrontation in eastern Europe. US officials have been saying that while they deal with matters in the Middle East and Asia, Europe with its economic leverage, should take the lead on Russia. But, while further punitive economic measures may hurt Russia, they will also cause damage to European commerce and industry.
Western governments are unlikely to be forced to take action by their electorates. The shock of seeing the Kremlin's ”green men“ in action abroad has palled after Crimea and, in the case of the Donbass, the Ukrainian government is suffering from ”crying wolf“. ”Russian forces have actually entered Ukraine“, said Mr Poroshenko in a statement. But, how many times has he and his ministers claimed that now? Indeed, the Ukrainian President called David Cameron earlier this month to say his artillery had destroyed an invading Russian tank column. Kiev, however, failed to produce any evidence of this triumph; the Russian foreign ministry described the claim as ”some kind of fantasy“.
Nato will hold one of its most important summits in its recent history next week and it is likely that a new set of sanctions will be imposed on Russia. But Putin knows that the Alliance will offer no military challenge and with his popularity climbing to 86 per cent from 55 per cent after the Ukrainian mission, this is a confrontation from which he cannot afford to back down.