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It is a month since Vladimir Putin's forces invaded Ukraine with nearly 200,000 men. One of the first cities to feel the full force of the invasion was Kharkiv in the north-east. Our correspondent Quentin Sommerville and cameraman Darren Conway have spent time with two fighters who have been at the front line since the very beginning.
This report contains some content some readers may find disturbing
The tale of Kharkiv is the story of the army that didn't fail, and an army that failed to win.
While Russia stumbles, Ukraine stands firm. Defying widespread expectations that it would collapse in short order, Russian forces have been unable to breach the Ukrainian army's lines around Kharkiv and have not managed to encircle the city.
Russia invaded at 05:00 on 24 February. The night before, 22-year-old Vlad and his brother-in-arms Mark, also 22, were at a fellow private's wedding. Columns of Russian tanks, howitzers, armoured vehicles and troop transports rolled across the border, just 40km (25 miles) away. Despite the long build up of Russian forces, the move came as a shock to the inhabitants of Kharkiv. Troops scrambled to defend the city.
When they learned of the attack, Vlad and Mark joined their battalion – the 22nd Motorised Infantry – and headed straight to the front lines. They have been there ever since. I have visited them there twice on the city's northern edge – a once pleasant suburban neighbourhood, which has now become a muddy battlefield strewn with corpses and burned-out Russian tanks and vehicles.
But it is sound, not sight, that is so jarring here. All manner of Russian artillery and missiles are fired at these positions almost continuously. When there is a respite in the shelling, or the roar of Russian Grad rockets, the silence itself comes as a shock. Ukrainian forces have lived under this terror for weeks now.
At a nearby command post, its windows all gone, broken furniture is strewn around. In an outbuilding, a belt-fed machine gun sits incongruously by a baby's pram. Children's climbing frames are surrounded by impact craters, and on one nearby abandoned house, a For Sale sign flaps in the freezing wind. Against the regular beat of Russian artillery outside, I ask Mark and Vlad what they are fighting for.
Vlad's reply is short and to the point, "For peace in Ukraine." Mark shoots him a glance, "My comrade says for peace in Ukraine," he laughs, then he swears and asks, "Who knows? These people came to our land. No-one was waiting for them here, no-one was calling them."
On that first day, one group of Russians made it into the centre, but were repelled after three days of hard, bloody fighting – with heavy casualties on both sides. The Russians were forced out beyond Kharkiv's edge.
A month on, while Russian missiles still strike at the city centre and at least half the 1.4m population have fled, there are neighbourhoods that remain untouched.
But, the city's eastern and northern residential neighbourhoods, which were largely intact when I arrived here three weeks ago, are unrecognisable. A tree has an unexploded Russian shell in its base; an apartment block has a 500kg bomb resting on its roof – if it had detonated, the whole building would have been brought down.
Mark and Vlad keep this grimness of war from family ears on the calls home they make most most days, just a couple of minutes each to mothers and girlfriends. So there is no mention of the dead bodies at the back door and in the next garden, no mention of the colleagues killed by Russian shelling, or of the tank commander who died the previous day. And nothing that could reveal operational details.
"Mainly we discuss when this will all end, when we can return to normal life, when everything is good and it won't be dangerous to walk outside," says Vlad.
A bank of phone chargers is connected to a generator in the building. The room where they sleep is warm and orderly. An elderly German Shepherd dog lives with them, she's traumatised by the chaos around her and moves from Mark to Vlad, soldier to soldier. A brief head rub and she goes to the next man seeking comfort from the noise and disorder outside.
The two men live everyday with Russians targeting their positions. Full-time soldiers, the Ukrainian army is their life.
The Ukrainian soldiers might have it rough, but the Russians seem to have been particularly unprepared for anything other than the shortest possible campaign in Ukraine. The corpses I have encountered in the snow have been poorly dressed for a winter campaign, and Ukrainian soldiers say they found the most meagre of rations with them.
Do they think of the soldiers on the other side, I wonder? Vlad says he has a message for them, "Run. Run away. Either you stay here in the ground or you go back home." He pauses but then adds, "Don't kill kids, destroy homes and families." This time it is Mark who is to the point, "Go back home while you are still alive."
The Russian war machine is a formidable adversary – but in the initial phase of the war, the Ukrainian military put into practice lessons learned from Russia's 2014 annexation of the Crimean peninsula, where the Ukrainians were found seriously wanting. Ukrainian forces however, are still significantly outmatched in numbers, technology and airpower.
So how have they held off the Russians so successfully?
Ukraine war: Quentin Sommerville and cameraman Darren Conway
A purported intercepted phone call, along with Western intelligence reports, may provide some of the answers. It is from a Russian commander in Mykolaiv, near Odesa in south-west Ukraine, to his superiors on 11 March. It was released by Ukrainian officials and has not been independently verified. It paints a picture of Russian misery and incompetence in the Russian campaign that both the US Pentagon and the UK's Ministry of Defence have, in part, detailed.
Troops lack basics such as tents and body armour – and are digging trenches in freezing ground to sleep. Two weeks ago, at another front line position in the city, I asked a young Ukrainian commander if his men slept in trenches. "Why would we sleep here when we can sleep in houses. The Russians sleep in trenches, but we sleep over there," he said, pointing to a well-heated house filled with men. He explained that the dead Russians had Kevlar body armour but many lacked the armoured plates that make the vest effective.
Mark and Vlad are well equipped. As we move through forward positions, there is ammunition and weaponry everywhere. Piles of rations and, in the kitchen, tea and coffee being made from a dark, cast-iron kettle. Inside their vehicles there are plenty of cigarettes – despite the familiarity with the chaos around them, many of the men chain-smoke.
When news comes over the radio that a colleague has been injured, an ambulance arrives within minutes and the casualty is covered in a heat blanket. He is bleeding, but is quickly stabilised. A Russian shell has peppered him with shrapnel and he has lost most of his fingers on one hand.
Hours later, as we head back to the rear, news comes over the radio that the soldier is stable and will recover.
The Ukrainians revel in their home-team advantage. They offer us biscuits and freshly delivered cakes from local factories. Their enemy has no such luck. There have been reports of Russian troops looting and foraging for supplies, villagers near Kharkiv complain that chickens and produce have been stolen.
A video of a captured Russian army cookhouse gives an unappetising glimpse of the meals served to troops. Servings piled high with onions and potatoes – all held together with congealed fat. Russian army rations – Meals, Ready-To-Eat (MRE) – with an expiry date of 2015.
When I met Mark and Vlad the first time, their commander gave me one of their sturdy green packs of Ukrainian daily rations – a leaving gift, he said.
There were 17 different things inside: wheat porridge with beef; rice and meat soup; beef stew; chicken with vegetables; pork and vegetables; crackers; biscuits; tea bags; coffee; blackcurrant drink; honey; sugar; black pepper; chewing gum; bar of dark chocolate; plastic spoons; moist wipes.
Ukrainian fortitude may be partly thanks to an unlikely suspect – Vladimir Putin.
In 2014, the Ukrainian army was in a terrible state. As it fought and failed to prevent the annexation of Crimea, its troops went hungry. Corruption was rife, training and equipment lacking and its chain of command unresponsive. Vlad and Mark's battalion was reconstituted the same year. The whole Ukrainian army underwent an overhaul – to make it ready for the next war with Russia.
Vlad and Mark, and almost every fighting man I have met on the front line over the past three weeks, have one thing in common – they have all fought in the eastern Donbas region. Some sport combat patches on their body armour with "donbasonia" written on them. Some of the patches though, we discovered later, were also symbols used by the ultra-right.
In the separatist Donbas enclaves of Donetsk and Luhansk, Ukrainian forces have been combat-tested for the past eight years. Between 250,000 and 400,000 Ukrainian men may have done tours of duty there since 2014.
"Ukraine is not the same country it was in 2014," one front line commander told me – echoing a sentiment that was repeated again and again to me in Kharkiv.
This has created a more professional army, and one with common purpose. An army that knew Russia wouldn't stop in Donbas or Crimea – and that a day of reckoning was sure to follow for the rest of the country.
In short, Ukraine isn't the pushover it once was.
War in Ukraine: BBC analysis
As many as 190,000 Russian troops have been deployed to Ukraine – with additional Chechen and Syria forces boosting their ranks. Ukraine's army stands at 100,000, but Kyiv claims it can rapidly mobilise significantly more.
And a month in, here in Kharkiv and across many other fronts in Ukraine, morale is strong among Ukrainian forces. "We are fighting for our land," Mark told me. What are Russia's mainly conscript fighters dying for? There are plenty of dead Russian fighters at various battlegrounds around the city. The Ukrainian dead, on the other hand, are quickly cleared away – but no official casualty numbers have been released.
Few of the Russian corpses appear to be ethnic Russians, instead they are ethnic minorities. White bands on their uniforms distinguish them from regular Russian troops. "These aren't real Russians," another Ukrainian fighter said as we passed bodies by the road. "They don't know why they are here," he said.
For the Ukrainians, this is seen as a good thing. Ethnic-minority Russian troops have weaker allegiance to Moscow, they say. One senior Kharkiv figure told me, "We don't fear Chechens, it's the Russians in Moscow restaurants who are afraid of them."
Kamil Galeev of the Wilson Centre, a US think tank, explores the condition of the Russian army. He suggests the troops are underpaid and undermotivated. Certainly, recruitment is a problem in Russia where dropping fertility rates mean there are fewer young Russians available to fight.
In Kharkiv, the winter snow and frost is beginning to melt. I join Mark beside his foxhole – a pit dug in the ground, on an embankment that is the front line. His boots squelch in the mud, the battlefield has become gooey, difficult terrain.
The thawing weather might not help Russia either – two weeks ago the temperature here was -13C, it is now eight degrees. As the mud deepens to grip boots, vehicles and kit, it becomes a trap for attackers and a boon for those defending the farmland around the city.
Further down the line, a soldier spots movement in nearby woods and opens fire. There is gunfire in response. "We have to move, there isn't enough protection here," says Mark "One hundred per cent, they will respond [with artillery]."
Sure enough, shells begin to fall only metres away and dirt is thrown up in the air. The shells land close enough to feel the shockwaves in your chest. Our team scrambles for cover under a nearby vehicle.
But Mark and Vlad seem untroubled. Everyone here told me the first three days were the worst. "This is much easier now," says the men's commander, who never once breaks into a run during the constant shelling, and hardly takes his phone from his ear, or the cigarette from his lips.
A quick glance over their shoulders to check where the explosion hits, and Mark and Vlad continue the conversation. "It's OK, you get used to it. Humans adapt to everything quickly," Mark says as another explosion punctuates his sentence.
What's going on right now, I ask, aware that cameraman Darren Conway is rolling. "They are working on our position," says Mark. "It's artillery," adds Vlad, with a nonchalant upward nod.
As the two men head back to shelter for a smoke and some tea, they pass the spent cases of US and UK-supplied anti-tank weapons. These, too, have been a decisive factor in this war. I have seen the aftermath of those missile strikes – at least a dozen rusting shells of Russian armoured vehicles, trucks and tanks.
But "the Ukrainian version is just as good," says another soldier, patriotically. Now is the time for once-sceptical Western governments to throw their weight, with more supplies and intelligence, behind Ukrainian resistance, another commander tells me.
The Ukrainian national anthem contains the following lines:
Our enemies shall vanish
Like dew in the sun
We too shall rule
In our beloved country.
Soul and body shall we lay down
For our freedom
There is little chance of Russian troops vanishing from Ukrainian soil.
Already there are reports that north of Kyiv, they may be digging and forming defensive positions, since their advance was stymied. And Russia, with nuclear and chemical weapons, as well as a range of sophisticated conventional weapons, has the power to escalate its bombardments of Kharkiv and other cities. It has done so before in Grozny and in Syria, and there, Russia with all its firepower proved that artillery requires little morale or motivation to be effective.
But Ukrainian forces, a month into this war, are satisfied they have defied expectations. With each week that passes, their chance of remaining independent grows, they believe. Russia isn't going anywhere, but neither are Mark and Vlad, nor the dozens of other Ukrainian soldiers I've met who say they are in this fight until the very end. Whenever that may be.
Follow Quentin on Twitter at @sommervilletv
This piece was amended after publication to reflect updated information about the soldiers' insignia.
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