Some observers have begun to ask whether a Ukrainian victory is conceivable. That depends on how victory is defined. It is President Zelensky's stated intention to recover all occupied territories as well as Crimea.

General David Petraeus, former CIA Director and commander of US military forces in Iraq, said he expected Ukraine to retake territory seized by the Russians since February, and "it's even conceivable they could retake Crimea and the Donbas," aided by growing resistance in occupied areas.

But that would take time and involve tough fighting, Petraeus told CNN. If that were Ukraine's goal, its supply lines would be stretched and its better units spread thin. In turn, Ukrainian forces would be vulnerable to counter-attacks.

Ultimately, Ukraine's battlefield success will depend on a continuing and expanded supply of Western hardware. Meetings in the next few weeks will determine what's in that pipeline, but inventories in several countries are dwindling.

US officials are also wary that Ukraine might overplay its hand. The US is still exceptionally cautious about sending Ukraine weapons that have a range of more than 80 kilometers (nearly 50 miles) and could therefore strike deep inside Russia. It has so far resisted Ukrainian requests for long-range Army Tactical Missile Systems (ATACMS) that have a range of up to 300 kilometers.

Some Western officials fear humiliation for the Kremlin might provoke an unpredictable reaction, even including tactical nuclear weapons.

A former NATO deputy secretary-general, Rose Gottemoeller, told the BBC this week: "I do worry about that kind of scenario at the moment... The goal would be to try to get the Ukrainians in their terror to capitulate."

Back in February, on the eve of the invasion, Putin warned that any country standing in Russia's way would face "consequences such as they have never seen in their history."

But Olga Olika, director of the Europe and Central Asia Program at the International Crisis Group, believes the Kremlin would not countenance such an escalation because "detonating weapons of mass destruction would provoke international retaliation, including, quite possibly, direct military involvement from NATO."