Ukraine only has around 45 days left before poor weather conditions hinder its counter-offensive against Russia, the head of the US military has warned.
General Mark Milley, speaking to the BBC along with with the head of the UK's Armed Forces, Admiral Sir Tony Radakin, said it was too early to say whether Ukraine's summer offensive had failed.
The chairman of the Joint Chiefs of Staff said: "That offensive kicked off about 90 days ago. It has gone slower than the planners anticipated. But that is a difference between what Clausewitz called war on paper and real war."
Gen Milley was referencing Prussian general Carl von Clausewitz, who is renowned mainly as a military theorist interested in the examination of war, focusing on the campaigns of Frederick the Great and Napoleon as frames of reference for his work.
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Gen Milley added: "So these are real people in real vehicles that are fighting through real minefields, and there’s real death and destruction, and there’s real friction.
"And there’s still a reasonable amount of time, probably about 30 to 45 days' worth of fighting weather left.
"So the Ukrainians aren’t done. This battle is not done. They haven’t finished the fighting part of what they’re trying to accomplish. So we’ll see, it’s too early to say how this is going to end. They at least have achieved partial success in what they set out to do, and that’s important. And then the rains will come in. It’ll become very muddy.
"It’ll be very difficult to manoeuvre at that point, and then you’ll get the deep winter. And then at that point, we’ll see where things go. But right now, it is way too early to say that this offensive has failed or not failed."
Admiral Sir Tony Radakin, speaking in the same BBC interview during a visit to Arlington in the US to lay a wreath on the Tomb of the Unknown Soldier, said: "Ukraine is winning and Russia is losing.
"And Ukraine is winning because the aim of Russia was to subjugate Ukraine and to put it under Russia’s control - and that has not happened and it never will happen.
"And that is why Ukraine is winning and Ukraine is making progress in its battle to regain its territory.
"It’s recovered 50% of the ground that Russia seized, and you’re seeing the international community also applying economic pressure and diplomatic pressure, and Russia is suffering because of that.
"And it's broken these rules and it’s paying the cost."
Ukraine's counter-offensive launched earlier in the summer with the aim of taking back Russian-occupied territory, but despite some advances, there have only been reports of some small tactical gains.
The UK's defence intelligence, however, has reported how Ukraine has made some advances in the southeast of the country, having pushed past Russian defensive lines east of the town of Robotyne.
Gen Milley added: "I think all the political leaders of Nato and Europe and the United States certainly, have said repeatedly over time in the last year, year and a half, that what does winning look like?
"At the end of the day, Ukraine must remain a free, sovereign, independent country with its territory intact.
"Russia, under the leadership of Putin, conducted an illegal, aggressive, military invasion of a country that presented no military threat to Russia. It was an illegal war of aggression by a very large country against a relatively small country, against their neighbour.
"If Putin's invasion is allowed to stand, then those rules, those principles, are for nothing.
"All of World War Two will have been fought for nothing.
"We're not fighting for Ukraine, and Ukraine hasn't asked anybody to fight for them.
"All they are asking for is help. They are asking for material help and training and ammunition, and that’s exactly what the international community is giving them in order to keep them free and allow them to defend themselves."
Asked what was going on in Russia as it seeks closer ties with countries such as North Korea, Admiral Sir Tony said: "I think it’s much more in the territory of desperation.
"It's a reflection of the catastrophic mistake that Russia made by invading Ukraine and it's a reflection of how few partners Russia has around the world, and where it needs to seek help, and it's also a reflection of the domestic situation in Russia.
"You've got to look at that the economy is under pressure, the sanctions are having a greater impact, it doesn't have a raft of international partners, it's lost half a million people that have fled the country, there's at least a million more that want to leave the country, it's struggling to have enough people to keep supporting the war, so this is an act of desperation.
"And that's why we need to focus on Ukraine, focus on the resolve and the will to fight of Ukraine, and that is why we continue to support Ukraine and we need to reflect on Russia’s desperation and see it for what it is."