KIEV - Ukraine's beleaguered forces have faced problems all of their own making as they've failed to retake separatist territories during a year of conflict - poor leadership, few supplies, bad coordination.
But the main reason Kiev has struggled to reclaim the rebel regions, some analysts in the West and Ukraine say, is that it has been facing off against thousands of regular Russian troops that Moscow poured in to bolster the insurgency despite blanket denials from the Kremlin.
"We are not talking about rebel forces, we are talking about Russian troops," Peter Felstead, an editor at the London-based IHS Jane's Defence Weekly, told AFP.
"This conflict would not be an issue were they not there. They are intrinsically involved in what is going on." Over the past year Moscow has been unrelenting in rejecting claims from Ukraine, NATO and Washington that it has sent troops across the unprotected border.
But Western and Ukrainian analysts back up Kiev's allegations that Moscow orchestrated the separatist rebellion of April 2014, and has been directly involved in fighting that has claimed more than 6,000 lives since.
The start of the conflict in the east came on the back of the ouster of Kremlin-backed leader Viktor Yanukovych by protestors in Kiev and Moscow's deployment of unmarked troops to seize Ukraine's Crimea peninsula.
In the wake of these seismic events that saw Ukraine lurch towards Europe and shattered Russia's ties with the West, analysts said Putin gambled on his men fomenting a rebellion to ensure the Kremlin's influence in its ex-Soviet neighbour.
"Their role is to put into effect Putin's will which is at the very least to create a buffer zone between Ukraine and Russia... and maybe to link up with Crimea," Felstead said.
The constant supply of weapons and ammunition over the Russian border has also allowed rebels to equip themselves better than their Ukrainian opponents, said Felstead.
'Thousands' of Russian troop
Although poorly equipped and trained, after months of fighting the Ukrainian armed forces did at one point seem to come close to crushing the rebellion over the summer.
They forced the rebels out of one key stronghold and nearly cut off the main insurgent-held city of Donetsk from the Russian border.
But suddenly, after weeks of government advances, the tables turned. That, experts say, was down to the arrival of thousands of Russian troops.
Some 3,000 to 6,500 soldiers crossed over the border, according to a report by Russian arms control expert Igor Sutyagin, published by British think-tank Royal United Services Institute for Defence and Security Studies (RUSI).
Ukraine claimed they then helped surround its soldiers trying to recapture the city of Ilovaisk in the Donetsk region, trapping them and killing hundreds over a few days.
"The presence of large numbers of Russian troops... has since become a permanent factor in the conflict," said Sutyagin, who was jailed in Russia on espionage charges before being handed to the West.
A senior Ukrainian security official told AFP on condition of anonymity that Kiev estimated the number of Russian troops in Ukraine peaked at around 15,000 in February during the fierce fighting for the town of Debaltseve.
Putin, however, claims that Ukraine's poorly trained forces are being beaten by a group of former "miners and tractor drivers" who have captured weapons abandoned by the Ukrainians.
But those on the other side fiercely disagree, saying Russia has sent in its latest weapons to bolster the rebels.
The separatists now boast "a bigger army... (tanks, armoured personnel vehicles, heavy artillery, missiles) than some countries in NATO and Europe," the US Embassy in Russia tweeted in February.
Oleksiy Melnyk, an expert from Kiev think-tank the Razumkov Center, alleged that at the height of the fighting they "used up to 150 tons of ammunition per day".
Governments in the West also claim that the separatist forces are using weapons that have never been given to Ukrainian troops.
These include a new version of the Soviet T-72 tank and the Tornado multiple-rocket launcher that was only delivered to the Russian army in 2011, according to the Ukrainian security official.
But Russian military might is not the only reason for Kiev's battlefield defeats.
The new government inherited a creaking military - a legacy of the Soviet era - that steadily deteriorated since Ukraine gained independence in 1991.
It comprised only 6,000 operational troops before fighting broke out last year, according to some estimates.
More than 50,000 government-controlled soldiers are currently engaged in eastern operations, but a deep "crisis in management" caused by a severe shortage of skilled commanders has led to poor decision making and sowed indiscipline among the ranks, said the experts.
Assistance from supporters in the Western world and volunteers has helped the army to restock essential kit such as uniforms, helmets and armour. But this "is only the start," acknowledged the Ukrainian official.