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Watch: 'Battle for Donbas' has begun, says Zelenskyy
Russia's war against Ukraine has entered a second, potentially decisive phase, with a new Russian assault on the east marking the biggest test yet for Ukrainian defenders.
Volodymyr Zelenskyy, Ukraine's president, used his nightly video address on Monday to declare "Russian troops have begun the Battle for Donbas".
He said: "A very large part of the entire Russian army is now focused on this offensive. No matter how many Russian soldiers are driven there, we will fight. We will defend ourselves."
The comments signalled a concentrated Russian offensive in the east is now underway, with both sides aware that whoever wins the battle could dictate the future direction of the war.
Russia needs to take Donbas to recover momentum
Ukraine has already dealt Russia a serious blow by forcing its troops to abandon an attempt to capture the capital Kyiv.
If Ukrainian forces can prevent Russia's soldiers from achieving their objectives in the east, it would significantly strengthen Ukraine's hand in peace talks - unless President Vladimir Putin, backed into a corner, chooses to escalate even further by unleashing a nuclear strike.
For Russia, by contrast, taking control of Donbas, which spans the regions of Donetsk and Luhansk, is vital to recover the momentum in a war that has been beset by setbacks, losses and failures for Russian troops.
A Russian success in the east would also likely embolden Putin to send his forces back north to finish a complete capture of the country, toppling the government in Kyiv. Western officials still believe that to be the ultimate objective of Russia's invasion.
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It means the stakes could not be higher.
Russia concentrating its forces in the east
Ukrainian commanders have been bracing for a new offensive in the east ever since Russia announced a major rethink in its war effort more than three weeks ago after losing the Battle for Kyiv - not that its defence ministry admitted any such defeat.
Thousands of Russian troops, tanks, artillery batteries and other weapons have withdrawn from around the capital and across the north of Ukraine, pulling back either into next door Belarus, a close Russian ally, or into Russia.
Since late March, they have been regrouping, stocking up on ammunition, food and other supplies and shifting down to reinforce positions around the Donbas region in the east and southeast.
The repositioning illustrates Russian military planners have given up for now on an attempt simultaneously to conquer the north, south and east of the country in the face of far fiercer than anticipated Ukrainian resistance.
Instead, learning from their mistakes, they are concentrating their superior firepower on a single part of Ukraine that has already been fighting Russia for the past eight years.
Read more: The war in Ukraine explained
The Donbas was the focus of Putin's first invasion of Ukraine in 2014 when his forces captured the Crimean peninsula before backing separatists who have fought Ukrainian forces along a line of contact there ever since.
Just three days before launching his second, far larger invasion of Ukraine on 24 February this year, Russia's president chose to recognise Donetsk and Luhansk as independent, with his troops then sent in to "maintain peace" in the territory.
Russian forces, though, have yet to achieve any major geography-changing advances in the region, with a large part of their effort focused on trying to take the port city of Mariupol. A pocket of Ukrainian resistance is still holding out against all the odds.
The eastern city of Kharkiv, in a region that sits next door to Luhansk and Donetsk has also withstood significant and sustained Russian bombardments.
By reinforcing its troops and firepower across the east, however, Russia will seek to take better advantage of its greater military strength compared with Ukrainian forces.
Ukraine's best troops preparing for a Russian onslaught
The Ukrainian side has benefitted from urban fighting conditions in towns and villages encircling Kyiv, using insurgency-style tactics to hit columns of Russian armoured vehicles and move clear.
If Russia is able to mass its land power in rural Donbas, supported by long-range missiles fired from within its own territory, its forces will prove a much tougher challenge to defeat.
In addition, Ukraine's military has not had the luxury of being able to pause operations, move out of the warzone, rest and resupply.
Its troops across the country have been fighting since day one of the invasion, fending off Russian attacks from multiple directions in a war of survival.
Unlike their opponents, they cannot move all of their manpower from the north to the east because the capital and surrounding areas still need defending.
That said, Ukraine's most effective, well-trained and battle-hardened troops are based in the east and will have been preparing for the next Russian onslaught.
Western officials have also cast doubt at the ability of the Russian military to regroup in as big a number of tanks and troops as desired and in as quick a time as needed.
In addition, Ukraine's ability to defend itself has only grown since February thanks to a combination of fighting spirit and an influx of increasingly lethal western weapons.
A skilled deployment of drones as well as anti-tank and anti-aircraft missiles will help to even the playing field, though Ukraine needs more tanks, jets and heavy weapons urgently.
Ultimately, the Battle for Donbas will likely see the heaviest and bloodiest fighting so far in a war that has already been relentlessly brutal, as both sides need to win.