Kyiv needs to deal Moscow a decisive blow before Western patience runs out

The removal of Prigozhin's mercenary force from Ukraine, which was implemented after the notoriously belligerent Wagner leader launched an ill-fated coup against Russia's military leadership, constituted another serious blow to Russia's military strength in Ukraine, as it left Moscow without the one military force that had proved itself capable of defeating its Ukrainian adversaries.

Kyiv victory slips away from the horizon

Yet, despite all these setbacks, the Russians still appear to be holding the extensive defensive positions they have established in the areas of eastern and southern Ukraine that they have occupied since the start of the conflict, raising serious questions about the ability of the Ukrainian counter-offensive to achieve its stated objective of liberating all of the territory captured by Russia - including the vital Crimea peninsula.

The latest confirmation that Ukraine's counter-offensive is not going as well as earlier predicted came at the weekend when US National Security Adviser Jake Sullivan conceded that the battle remains "hard going" for the Ukrainians.

"We said before this counter-offensive started that it'd be hard going, and it's been hard going," Sullivan said in an interview with ABC's  "This Week" anchor George Stephanopoulos.

"That's the nature of war, but the Ukrainians are continuing to move forward."

"We're continuing to supply them with the necessary weaponry and capabilities to be able to do that and they will keep attempting to take back the territory that Russia has illegally occupied." 

Read more: Pentagon leaks show Western resolve in Ukraine war

Increasing Republican opposition

Sullivan was also critical of deepening Republican opposition to the Biden administration providing a "blank cheque" in terms of its support for Ukraine. 

As Florida Governor Ron DeSantis, a contender for the Republican ticket in next year's presidential election contest, remarked last week, "I think that right now you have an open-ended blank check. There's no clear objective for victory. And this is kind of dragging on and on."

Sullivan dismissed the criticism, arguing, "The American people have really hung in there and supported the Ukrainian people."

Even so, one of the key reasons for Ukraine's faltering offensive has been a lack of weaponry, especially top-end warplanes such as US F-16 fighters.

Ukrainian President Volodymyr Zelensky has consistently highlighted a lack of warplanes as hindering his forces' ability to advance deep into Russian-held territory. As Zelensky warned in June, without effective air cover against the Russians, "a large number of soldiers will die."

Despite pledges from the US and its allies to supply Ukraine with F-16s, they are unlikely to arrive before the end of the year, by which time the fighting could be over.

Read more: Washington manages expectations over F-16s approval for Kyiv

The desire to limit Ukrainian casualties, as well as shortages of other military equipment, such as armoured vehicles, have forced the Ukrainians to adopt a cautious approach, especially in the south, where the battleground is mainly open farmland, making it extremely difficult to advance without adequate protection.

Western military intelligence also believes that, despite the setbacks the Russians have suffered, they have, nevertheless, succeeded in constructing a highly effective network of defensive lines involving tank traps, barbed wire, minefields, bunkers and trench lines.

An official at Britain's Ministry of Defence recently described the Russian fortifications as "some of the most extensive systems of military defensive works seen anywhere in the world."

The difficulties the Ukrainians have experienced in their efforts to break through the Russian lines have prompted Washington to take the controversial step of providing Kyiv with cluster bombs to help them clear the Russian defences.

Many countries oppose the use of cluster bombs because of their devastating impact on the battlefield. Still, the US has deemed their use necessary to help the Ukrainians overcome Russia's military superiority. 

In recent days Ukrainian forces have begun using the weapons, with White House officials reporting that they are being used "appropriately" and "effectively".

Even so, the lack of progress being made by Ukraine's forces will be a matter of grave concern for Zelenksy, as any failure by Kyiv to inflict a decisive defeat against the Russians is likely to lead to an erosion of support among Western leaders for the Ukrainian war effort.

Many Western leaders, including US President Joe Biden, want to see the war end as soon as possible and believe a negotiated solution to the conflict will become inevitable if the Ukrainian counter-offensive fails to make any significant progress.

The Biden administration has already laid the groundwork for a possible deal with Moscow after it was recently revealed that a delegation of high-ranking former US diplomats had met in New York in April with Russian foreign minister Sergey Lavrov to discuss how to end the war.

The Ukrainians hope that, by defeating the Russians on the battlefield, they will be in a stronger position to dictate terms at any future peace talks. But if they fail to achieve their objectives, they could once again find themselves at the mercy of their Russian occupiers.