Ukraine has now been under almost constant bombardment from Russia for more than four months, but the defining months could be just ahead, analysts say.
"The war is on something of a knife edge at the moment," Michael Clarke, former director of the Royal United Services Institute, told ABC News.
"Ukrainians are in a very difficult situation because they're running out of ammunition, they're being outgunned on the battlefield.
"They have to hang on desperately for the next two to three months before the pendulum will begin to swing in their favour."
Russia is now thought to occupy around 20 per cent of Ukrainian territory, with fierce fighting taking place in the eastern Donbas region of the country.
After a long battle Ukrainian forces were forced to retreat from the city Sievierodonetsk, one of the few remaining strongholds in the key Luhansk region.
"The question we're all asking ourselves is how much territory might they be giving up in these next two to three months," he said.
Increased air attacks in major cities have claimed dozens of lives in the past week, with the strike on a busy shopping centre in Kremenchuk described by world leaders as a "war crime".
On early Friday morning local time, Russian missile strikes in and around Odessa, including on an apartment building, killed at least 19 people, including two children.
Experts believe these assaults have been designed to induce fear, suffering and surrender – but with renewed arms, aid and backing from the West, Ukraine could still ultimately win this war.
However, the death and destruction could continue or even escalate in the months ahead, with many predicting the war will carry on for years to come.
Ukraine must also find the firepower to make a difficult switch from "defender to offender" to regain lost territory.
"It will take them at least six months to put themselves into that position and they would still need a lot of Western help and Western equipment," Professor Clarke said.
"But it's not impossible."
Ukraine faces a long and 'generational' conflict
Defence analysts paint several different scenarios for how this period of intense fighting might conclude, but imagining what "victory" for either side might look like is more complicated.
"This is not the war which can be ended in several weeks," said Maria Zolkina, a defence analyst from the Kyiv-based think tank, Democratic Initiatives Foundation.
"The best-case scenario is that the hot phase will be concluded by the end of this year … but this is a very optimistic scenario."
For Russia, the effect of sanctions will continue to bite, military losses will mount and morale issues could deepen, while Ukraine expects to be bolstered by Western weapons over the next few months, she said.
"This balance will be more in favour of Ukraine with every coming month."
History shows that war is protracted and "never goes to plan," Rod Thornton, from the Defence Studies Department of King's College London, told ABC News.
"[World War I] was supposed to be over by Christmas 1914 – it dragged on for years.
"Hitler built a German army, basically designed as a blitzing army which would get wars over with very quickly.
"But [World War II] dragged on for about five years, so wars tend to drag on much more than you would think."
In Ukraine, periods of intense fighting could come in "fits and starts" for decades to come, Professor Clarke said.
"What we're seeing is a generational struggle over the independence of Ukraine, because Russia, under this present regime and [President] Putin … their view is that Ukraine has no right to exist," he said.
"I would love to be wrong about that."
Ukrainian and Russian victory are two different ideals
Russia's attack will only come to an end when the reign of Russian President Vladimir Putin comes to an end, Professor Clarke predicts.
"This war lasts as long as Putin lasts and probably some time into his successor," he said.
"Putin and all the people around him believe that Ukraine should be erased from the map and reabsorbed into Russia — that desire will remain the case for the next generation or so.
"He will only ramp it up until he is removed, and his removal might take place in two or three months, or it might be three or four years, there's no way of telling."
With every month that passes, the more difficult this war becomes for Russia, he warns.
"The sanctions will get worse, the isolation will get worse, the Russian economy will get worse … and believe me, Putin will not lose gracefully."
In February, the Kremlin initially described the invasion of Ukraine as a "special military operation" in which it was attempting to "de-Nazify" the country.
After their withdrawal from the north of Ukraine and their failed attempt to capture the capital, Moscow claimed its focus was liberating the territories in the mostly Russian-speaking Donbas region.
For Ukrainians, the idea of victory is about more than just regaining this territory, Ms Zolkina said.
Her organisation carried out research on the topic in Ukraine in recent months.
"When we ask people, 'What does this victory mean? How do you perceive it?' There are two opinions," she said.
"Forty-one per cent said that the victory of Ukraine is the complete withdrawal of Russian troops from the entire territory of Ukraine.
"Another 41 per cent had a more radical position, that the victory of Ukraine is the full, complete destruction of the Russian army and collapse of the Russian state.
That fight for "victory" in the coming months is shaping up to be bloody and bitter, experts warn.
The next phase is critical but could be more brutal
Russian forces are gaining at most one to two kilometres per day in the Donbas region, according to analysts including Professor Clarke.
"Their only technique is to bombard ferociously the ground in front of them, turn everything into rubble and then walk in on top of the rubble," he said.
"They've come up against a reasonably well-trained and very determined Ukrainian force who can fight in the rubble, then they're getting pushed back.
Much of the territory under Russian control in the south and east of the country was taken in the early days of the war while Ukraine set up a front line, Ms Zolkina said.
"At the very beginning of large-scale invasion, the Ukrainian army in Donbas consciously stepped back and organised the defence line around Sievierodonetsk, Lysychansk and Popasna, and at these defence lines the battles are going on for four months."
The Donbas is made up of two regions, Luhansk and Donetsk, where there has been some Russian occupation in parts since 2014.
In recent weeks, there has been a fierce battle for the city of Sievierodonetsk and Lysychansk is now one of the only towns in the Luhansk district still under Ukrainian control.
Donetsk's regional military administration claims that 45 per cent of its province remains under the control of Ukrainian forces.
"It took them six weeks to actually take Sievierodonetsk and that's a small city, 100,000 people before the war," Professor Clarke said.
"Now they're moving to Lysychansk, which is on the other side of the river — that will take them at least two weeks and may take them a fair bit more."
"It's moving at snail's pace."
Professor Clarke warns that as Russian forces face increased difficulty in gaining territory, they could begin to level even more ferocious assaults on civilians.
He said Russian missiles would not just be aimed at Ukraine's supply lines.
"They will hit civilian targets, deliberately aimed to kind of try and break down the Ukrainian people's morale."
This is a tactic to pressure Ukraine into negotiation, Ms Zolkina says.
"It is to show the Ukrainian public that there is no safe place inside Ukraine," she said.
"It is to make people demoralised, to feel unsafe, and to break the spirit of resistance … to make people pray only for the end of the conflict, whatever it means.
"The real challenge for us in Ukraine … we are dependent on our Western allies in weapon supply."
War effort dependent on Western support
In the war of attrition, Russia is winning by constantly mounting fierce air attacks and much Western support is needed to turn the tables, Dr Thornton believes.
"We are getting very much towards the stalemate … it's basically a slugfest of artillery versus artillery," said Dr Thornton.
"I can't see it shifting in the months ahead, because there isn't going to be the capability and the energy from either side, Ukrainian or Russian, to actually push the boundaries.
After a historic summit of NATO this week, world leaders have vowed to back Ukraine for the long term.
The US, EU and the UK have recently announced renewed funding and arms packages, while Ukrainian troops are receiving specialist training in England.
If this level of support continues, and powerful weapons make their way to the Ukrainian front line in the next few months, the country is capable of regaining lost territory, Professor Clarke believes.
"To actually throw the Russians out big-time requires the Ukrainians to be re-equipped, to reorganise themselves and to command themselves differently in a much more centralised way," he said.
"Whether or not it'll happen, rather depends on us in the West and how much we support the Ukrainians over the longer term."
"The mood music from NATO is certainly good but whether that turns into a practicality is another question … especially as leaders face troubles at home."
Winter could bring new difficulties for Ukraine and potential tactical advantages to Russian forces.
"If fierce battle is still taking place in winter, it could pose a real challenge to Ukraine," Dr Thornton said.
"The Russians will want to hit sites of heating [power stations] so that the people themselves come under the pressure,
"They get cold, they put pressure on their own government in Kyiv and say, 'stop this war because we're dying of cold'.
"I think the Russians are waiting for winter because that's what they feel, that from a psychological point of view they can have a greater effect."
Ms Zolkina believes Ukrainians will remain staunchly in favour of fighting for their country.
"Our research shows 90 per cent of people believe Ukraine can and will win this battle," she said.
The challenge lying ahead is really for Western nations, warns Professor Clarke.
"This is war and peace with the future of liberal democracy at stake — the risks are high, the stakes are high," he said.
"It's been relatively easy so far – this is where it starts to get tough."
