Much will now depend on whether Ukraine can muster enough troops and build sufficient defensive lines to hold up the Russian assault.

The Russian battle plan is not yet clear, but it appears to be guided partly by the landscape.

The eastern salient straddles the Siverski Donets river, the main watercourse running through the eastern battlefields. The right flank of the drive towards Lypstsi is protected by the Kharkiv reservoir.

It is possible that the ultimate intention is to eventually cut the Ukrainian lines of communication between Kharkiv and Kupiansk, the ridgetop town that Russian forces in the east have been fighting towards for months.

This may not be the last cross-border assault.

Kirillo Budanov, Ukraine’s military intelligence chief, said on Tuesday that Russia had committed all the troops available for the Kharkiv operation but that another reserve existed that might attack in the Sumy region, which borders Kharkiv to the west and north.

An attack here could threaten to cut Kharkiv’s main supply road from Kyiv and further stretch the Ukrainian defenders, a prerequisite for besieging the city itself.

Creating such a threat may be an objective in itself. The CDS think tank said in its report predicting the attack: “Their goal is to distract the Ukrainian Defence Forces command and prevent the use of reserves, especially strategic ones, in more critical areas.

“By launching a limited offensive in Kharkiv, they aim to threaten a breakthrough towards the city and tie down a significant portion of the Ukrainian Defense Forces there.”

Actually assaulting Kharkiv – Ukraine’s second-largest city, which has already withstood one siege and has been effectively turned into a fortress – would be massively ambitious.

Russia failed to surround the city even at the height of its first attempt to capture it in April and March 2022.

Fighting street-to-street, as in Mariupol or Bakhmut, would take months and require a vast commitment of men, equipment, and ammunition.

Speaking in the days and weeks before Russia’s attack on Vovchansk, Western officials and well-connected Ukrainian and foreign military analysts unanimously insisted no Russian reserve of that size exists.

“No one has seen it,” said Michael Kofman, a senior fellow at the Carnegie Endowment and one of the most trusted war analysts.

So if Kharkiv is a diversion, where is the real prize?

The Kazeny Torets is a slow-moving, overgrown river that joins the Siverski Donets about 100 miles downstream of the border.

Strung along its valley lie the road and rail junctions of Kostiantynivka and Druzhkivka; the provisional regional capital of Kramatorsk, which is also the largest town in Ukrainian-held Donbas; and Sloviansk, the town where the war began in 2014.

The potholed H20 motorway and a parallel arterial railway follow the river’s eastern bank, linking the cities into a single, sleepy agglomeration.

Together, these towns and their transport links make up the economic and strategic heart of the Ukrainian-held Donetsk region.

If the valley falls, Donbas falls. And Russia has made no secret of its ambition to capture it.

General Valery Gerasimov, the chief of the Russian general staff, has previously mounted offensives consisting of several attacks up and down the front line, only one of which is the main effort.

“And the Russian military strategy has been fairly consistent, their operations are focused on taking Donetsk region first, and expanding territorial control in Kharkiv and Zaporizhzhia,” said Mr Kofman.

Serhii Kuzan, a former Ukrainian defence minister adviser and current chairman of the Ukrainian Security and Cooperation Centre, concurs: “I believe they will use the reserves they have to strengthen the groups in the Donetsk region and to continue their forward movement in this area, not to make new groups.”

“That makes it important for us to counteract them now because it will make them less threatening for further movements.”

Vladimir Putin has been trying to conquer the Donbas, the region comprising the provinces of Donetsk and Luhansk, for ten years.

His first attempt was in April 2014, when a few dozen mercenaries and thugs seized control of Sloviansk.

The second serious attempt came in the spring and summer of 2022, when Mr Putin redirected his forces following the failed attempt to take Kyiv.

The Russians never gave up on their goal. They spent tens of thousands of lives over nine months to capture Bakhmut, the city controlling the next river valley to the east of the Torets, last year.

And in February this year, after five-months of costly fighting, they captured Avdiivka, creating a major breach in the old fortified line built between 2015 and 2022.

Those developments set the stage for what may become Russia’s main effort this summer.

The outline of Russia’s Donbas battle plan is already visible. “Their [the Russians] strategy is aimed at seizing the main transit hubs and population centres of the region,” said Mr Kofman.

He added: “They are advancing in a predictable straight line right now from Avdiivka towards the main roads that flow from Pokrovsk, and to Chasiv Yar from Bakhmut, in order to threaten Slovyansk and Kramatorsk.”

At the time of writing, the Russians have already pushed several miles beyond Avdiivka, but are still at least 17 miles from Pokrovsk itself.

But they are within six miles of the T0504 road linking Pokrovsk and Kostiantynivka, a critical Ukrainian supply route.

From the Bakhmut direction, they are advancing on Chasiv Yar, a tiny town perched on the upland between the Bakhmutska and Kazeny Torets valleys.

Capturing it would provide a foothold on the plateau, putting Russian field artillery in range of the Torets valley towns and giving their FPV drones free range to terrorise the H20.

They would have to fight only a few more miles along the ridges before they were looking directly down into Kostiantynivka and Druzhkivka.

Occupying the high ground and cutting the T0504 could lead to the isolation or complete capture of Kostyantynivka.

That, in turn, would “significantly impair the defence forces’ ability to maintain the frontline in the southern part of Donetsk oblast,” the CDS forecast said.

Russia’s exact plans and capabilities will only become known as the battle progresses. What is clear is that it is testing Ukraine along the entire length of the 800-mile front line.

In the extreme south, a tiny Ukrainian bridgehead on the left bank of the Dnipro river continues to hold out against Russian efforts to crush it.

In the Zaporizhzhia region, villages already flattened during Ukraine’s counter-offensive last summer are in danger of falling into Russian hands again.

Small Russian attacks here are making little progress, but they put further pressure on stretched Ukrainian forces struggling to fortify the Donbas.

Although unlikely, a breakthrough here could threaten supply lines from central Ukraine into Donbas.

Assaults continue against the Ukrainian front around Kupiansk in eastern Kharkiv region.

Advances here could open the way to Kharkiv city and allow Russia to threaten the Kramatorsk-Sloviansk agglomeration from the north.

Russia’s strategy and goals are likely to be subject to adjustment and adaptation.

If the operation in Kharkiv makes more progress than expected, they may reinforce it and go for the ambitious goal of Kharkiv itself.

In Donbas, Chasiv Yar is defended by a steep incline and a canal, and the Ukrainians have rushed in reinforcements.

The Russians may be willing to bypass the town, provided they can gain the heights.

There is also a likelihood that Russian commanders will seek to encircle the defenders of the Toretsk area between Avdiivka and Bakhmut rather than push on westwards.

Russia’s Operational Group Centre is believed to have been tasked with preparing the ground for such a move.

In the southern corner of the front, Russia’s Operational Group South is attempting to break through to the town of Kurakhove.

But drawing arrows on maps is easy. In reality, both sides will face grave challenges in this summer’s fighting.

Observation drones, satellites, and electronic eavesdropping mean the battlefield is almost entirely transparent. Massing troops for a major attack is risky.

Achieving surprise is impossible, as the accurate prediction of a May attack on Kharkiv makes clear.

Russia has so far been able to replace losses in men and equipment and has an advantage in ammunition.

But it has also struggled to operate above company level and has never managed to turn incremental advances into real strategic breakthroughs.

Its last attempt at manoeuvre warfare, when Russian generals tried to envelop Avdiivka with two pincer movements in October, was a costly failure.

The battle was won only five months later through grinding frontal assault that cost Russia at least 13,000 casualties.

Gains since then have been achieved in similar manner and at similar cost.

Speaking last week, Mr Kuzan claimed the Russians were suffering 1,000 casualties a day in their efforts to make progress towards Pokrovsk before new American ammunition arrives.

He said: “There are huge losses in manpower and this shows the attacks are very intensive. [Previously] we experienced attacks by Russians in seven directions, but now the only things we see are attacks in one or two directions, and this picture shows that Russians are also exhausted.”

Nonetheless, the Russians have made significant and dangerous adaptations to the battlefield, said Justin Crump, CEO of Sibylline, an intelligence consultancy.

Glide bombs – ordinary air-dropped bombs fitted with simple wings and guidance systems that allow them to be launched from safety and glide to precise targets – were decisive in Avdiivka and have become a significant part of Russia’s overall fire advantage.

Secondly, says Mr Crump: “The Russians have improved their sensor-shooter loop for support. The time from identifying a target to striking it with, for example, artillery or glide bombs is now much shorter than seen earlier in the conflict.”

In March, the Russians in separate incidents achieved their first ever kills of a Himars launcher and a Patriot missile battery.

Both vehicles were destroyed near Pokrovsk after drone operators quickly fed targeting coordinates to ballistic missile batteries.

Russia is also beginning to see results, albeit patchily, from strong investment in electronic warfare against Ukrainian drones.

Last, a number of Mad-Max-esque vehicles have appeared on the battlefield since the start of the year.

“Russian vehicle modifications can look ludicrous but have often worked well for the specific problem they have encountered – for example the ‘turtle tank’ or ‘tank barn’,” said Mr Crump.

“This sacrifices almost all features of the tank in exchange for providing effective protection from FPV (First-Person View) drones, which Ukraine is currently relying on to make up for artillery shortages.”

The biggest questions hang over Ukraine and its allies.

The six-month hiatus in US arms supplies, a delay in mobilising more men and a lack of urgency in digging defence lines have resulted in a moment of crisis the Russans are now trying to exploit.

Congress passed the $60 billion (£48 billion) assistance bill last month, and urgently needed assistance like shells should be reaching the frontline already.

But that alone will not change the trajectory of the war.

“There’s been sort of this perception that, you know, we’ve now passed the supplemental [Ukraine aid bill] so the worst is over. Well, that’s not really the case,” said Mr Kofman.

“In reality, the challenges that the Ukrainian military has will take months to address and the supplemental and Western support is not a magic wand.

“So the question is: ‘Will Ukraine be able to stabilise the frontline over the coming months?’ We are still very much in a period of vulnerability for the Ukrainian armed forces.”

The most basic problem, said Andriy Zagorodnyuk, a former defence minister and chairman of the CDS think tank, is also the easiest to address.

“The way they attack is this: they send a group of 10 or 15 or 20 people, with three or five or six armoured vehicles and tanks,” he explained.

“The idea is they attack in one location usually early in the morning. So they pull the tanks up in a staging area then during the night they throw them closer to the front.

“They select the weakest link in the line using UAVs (Unmanned Aerial Vehicles), then they attack. Usually we see that through our own UAVs, and send UAVs and artillery to destroy them.”

But where shells are tightly rationed, or where there is more than one attack in a day, the Ukrainian gunners quickly run out of ammo. The Russians overrun a village or treeline, consolidate, and the process begins again.

The resumption of American ammunition supplies is not expected to give Ukraine shell parity with the Russians, but it will make a critical difference to breaking up those small attacks.

Mr Zagorodnyuk added of recent Russian advances in Donbas: “This is basically it. Everything else is secondary. We could easily have stopped them.”

“I spoke to guys from the front yesterday who said if we had enough ammo all of those attacks would be unsuccessful.”

Then there is the belated approach to digging defensive lines. Put bluntly, the Ukrainians do not seem to have an equivalent of the Surovikin line that defeated their offensive last year.

Mr Butusov reported on Monday that the Ukrainians in the Kharkiv region were having to dig new positions on high ground because the initial defensive lines had not been created at naturally advantageous positions.

One angry officer told the BBC that the Russians were able to simply walk across the border near Vovchansk because no one had laid minefields.

Building such defences while simultaneously fighting to hold up the Russian offensives will be a major challenge.

But most critical of all is the troop shortage.

“They have not been replacing their losses since the offensive of 2023, which over time led to a deficit of manpower at the front line, especially infantry,” said Mr Kofman.

Anyone who has visited the front or spoken to soldiers serving there is familiar with the result: under-strength units trying to hold broader sections of front with fewer soldiers of poorer quality.

The most effective brigades are reliant on a dwindling hardcore of experienced and motivated fighters to maintain combat effectiveness, many of whom are exhausted after two years of constant fighting.

Ukraine’s government last month approved new laws lowering the conscription age from 27 to 25 and tightening punishments for draft dodgers, but the move is belated and it is likely to take months to recruit, train, and deploy enough men to stabilise the situation.

There are other issues to address.

There is still a shortage of UAVs, air defences are badly stretched, and significant issues are emerging around the maintenance of the diverse assortment of kit Ukraine’s allies transferred in the first two years of the war.

Russia’s attack on Kharkiv is designed to exploit those weaknesses. The outcome of the battle will depend on how effectively Ukraine and its allies address them.