This area includes cities like Donetsk, Luhansk and Mariupol.
As well as failing to achieve his military objectives, President Putin also appears to have failed in his aim of stopping further expansion of NATO, as both Finland and Sweden have applied to join the military organisation.
So, how big is Ukraine’s army - and how does it compare to Russia’s military?
How big is Ukraine’s army?
According to the International Institute of Strategic Studies (IISS), Ukraine had 196,600 active military personnel when the invasion began on 24 February.
This split into:
- 125,600 ground troops
- 35,000 air personnel
- 15,000 naval troops
- Another 900,000 reservists
These numbers have been bolstered as men aged between 18 and 60 have been conscripted and other civilians have volunteered to bear arms.
Ukraine has since announced plans to arm a further one million citizens.
The country also has paramilitary groups fighting Russia, such as the controversial far-right Azov Battalion.
The latest official estimate from Ukraine (from 16 April) is that it’s lost 3,000 soldiers.
The Russian defence ministry put Ukraine’s troop losses at 23,367 as of 16 April.
While no new overally estimates have come out of Moscow since then, it says it captured 1,730 Ukrainian troops when Mariupol fell following a three-month siege.
Russia also says it has downed 134 Ukrainian aircraft, 2,269 tanks and 987 artillery guns since the invasion began.
None of these claims have been independently verified.
Most of Ukraine’s firepower is land-based, with the country boasting more than 2,000 tanks, 1,960 artillery pieces and 2,870 armoured vehicles at the time of Russia’s invasion.
Troops also have received Nato lethal aid, including 10,000 missiles and 120 armoured vehicles donated by the UK, 500 Stinger surface-to-air missiles from Germany, and 90 howitzer artillery pieces, 120 Phoenix Ghost attack drones as well as $1.6 billion in support from the US.
President Joe Biden has since asked Congress to approve a further $20 billion (£16.2 million) in military aid.
Ukraine’s air power consists of 146 attack aircraft and 42 attack helicopters, while its navy has just two warships.
It remains one of a handful of countries to have given up its nuclear weapons, meaning it has no deterrent to threaten Russia with.
Given the country has been fighting so-called Russian backed separatists in the East of the country since 2014, its armed forces are well-versed in combat.
However, with a defence budget of between $2bn to $5bn, it does not boast the capabilities of its powerful neighbour.
How big is Russia’s army?
Russia’s military vastly outguns Ukraine’s, with a budget of between $40bn to $65bn.
The UK Ministry of Defence (MoD) says Russia’s defence spending doubled between 2005 and 2018 as the country sought to modernise its army.
These are its capabilities:
- 900,000 active military personnel across land, sea and air
- 280,000 soldiers
- 165,000 air personnel
- 150,000 naval troops
- Around two million reservists.
In an interview with BBC Radio 4, Professor Michael Clarke - a security expert and former director of the Royal United Services Institute - estimated 40% of the active Russian army consists of conscripts.
Vladimir Putin previously denied conscripts were fighting in Ukraine, but his defence ministry has since admitted some have played a role in the invasion.
Russian men between the ages of 18 and 27-years-old have to serve for a 12-month period every year but are not supposed to see frontline action under Russian law.
A new draft took place on 1 April.
The number of troops Russia had in and around Ukraine at the time of the invasion was estimated to be 190,000, as well as several hundred mercenaries from the Wagner Group.
It’s thought Russia has called up more troops from Chechnya, Eastern Russia and Syria to further bolster its forces.
Close Moscow ally Belarus has so far not officially committed troops to the conflict, although it has massed troops close to the border.
Having retreated from the Kyiv region in early April, Russia began an offensive in the Donbas on 18 April.
Its military leadership has also not ruled out forming a land bridge across southern Ukraine to the Kremlin-backed breakaway region of Transnistria in Moldova.
The MoD says the Russian force in eastern Ukraine includes remnants of the units that mounted attacks on Kyiv, many of which have merged.
An assessment from the Institute for the Study of War - a military think tank - said the new offensive was “unlikely to be dramatically more successful”, partly as a result of redeployed forces not getting enough time to regroup.
Russia has so far struggled to make major inroads into the Donbas as a result of Ukrainian counterattacks - particularly around the city of Kharkiv - resistance from the port city of Mariupol and its own inefficiency.
Ukraine claims to have killed almost 28,700 Russian personnel (over 15% of the initial invasion force).
The UK Ministry of Defence estimates a third of Russia’s February force has been lost.
Russia has only officially admitted to losing 1,351 soldiers - a figure given on 25 March, just over a month into the invasion.
It’s impossible to independently verify any of these claims.
Casualties are believed to include 12 Russian generals.
It’s thought they have been killed in vulnerable forward positions as they attempted to improve communication with frontline troops and bolster low morale.
Russia’s land-based capabilities include more than 13,000 tanks, just under 6,000 artillery pieces and close to 20,000 armoured vehicles - although not all of this firepower is being thrown at Ukraine.
Ukraine claims to have destroyed 1,263 tanks and 596 artillery pieces and 3,090 armoured vehicles.
Russia’s main advantage is its long-range weaponry, with the country possessing more than 500 land-based ballistic missile launchers.
It claims to have used air-launched Kinzhal hypersonic missiles for the first time - something US President Joe Biden confirmed on 21 March.
But the UK Ministry of Defence said that the weapon is still “developmental” and that its use was “highly likely intended to detract from a lack of progress in Russia’s ground campaign”.
Russia has continued to launch missile strikes across Ukraine despite its focus on the country’s east.
As well as key infrastructure, projectiles and artillery shells have also hit civilian areas, including flat blocks.
When it comes to airpower, the country has 1,328 attack planes and more than 470 attack helicopters.
Ukraine claims to have downed or captured 204 aircraft and 168 helicopters.
On 27 April, the UK MoD said the Ukrainian airforce was still in control of the “majority” of its airspace.
At sea, its 74 warships and 51 submarines dwarf Ukraine’s tiny fleet - although Moscow has only deployed around 20 naval vessels to the Black Sea, according to the UK government.
Russia has had plenty of battlefield experience over the last decade as a result of its involvement in Syria’s civil war, where it has propped up the regime of Bashar al-Assad.
It has also sent forces to Libya and invaded Georgia back in 2008.
However, this experience appears to have counted for little in Ukraine given what the MoD has labelled as strategic planning and operational execution “failures”.
The Ministry of Defence believes Russia’s military is now “significantly weaker” as a result of its invasion of Ukraine.
Part of the reason for its failings could be corruption.
Professor Michael Clarke told the BBC it runs “deep” within the Russian ranks and said the fact that most of the army believed it was on exercise in the run up to the invasion meant it was not battle ready.
“When they’re on exercise, they agree to all sorts of things that aren’t really true - they tick the boxes for maintenance that [isn’t] really there and they sell their equipment because they’re paid so little. And so the forces that have gone over the border, actually, are extremely unprepared,” he said.
Russia’s biggest threat to Ukraine and the rest of the world is its stockpile of nuclear weapons, the mere existence of which acts as a deterrence to directly engaging in warfare with the country.
Vladimir Putin heightened the alert level for Russia’s nuclear forces at the beginning of March.
How big are the UK’s armed forces?
According to the latest publicly available assessment by the Ministry of Defence (MoD), the UK’s full-time armed forces consist of more than 159,000 personnel.
This figure includes 76,300 ground soldiers, 29,960 air personnel and 29,136 naval troops.
Some of these troops are currently on deployment with Nato in Eastern Europe and the Baltic states amid fears Russia could be planning to invade other neighbours and former USSR states.
The UK actually has a target to reduce its land-based forces to 72,500 by 2025, meaning it would have 30,000 fewer troops than it did in 2010
At present, it has more than 32,000 reservists.
How big is the US army?
Few countries can match the might of the US military machine.
According to the latest estimates from 2020, the country spends more than $700bn annually on its armed forces.
In 2020, this was 37% of the world’s entire defence spending.
The US is believed to have over one million active ground troops, 300,000 naval personnel and 300,000 air troops.
How big is the NATO army?
The defence organisation NATO has around 3.5 million active and reserve personnel it can call upon should one of its members be attacked.