It's been a week since Ukraine's stunning invasion of Russia, and Volodymyr Zelensky's adventurous move appears to be paying off - at least for now.
This is what Frank Gardner says in the comments. He is the BBC's security correspondent.
The Kremlin is humiliated, Kiev's morale is on the rise, and there are signs that Russia will have to move some of its forces from the Donbas south involuntarily.
But the longer Ukrainian forces remain in Russia, the greater the risks.
Perhaps the biggest of these is that Russia will deploy enough forces to surround and cut off Ukrainian forces before they can withdraw.
Major losses or mass captures would wipe out any recent gains in boosting Ukrainian morale.
A senior British military source, who asked not to be named, said he was impressed by Ukraine's use of "combined weapons", ie. from Russia's inclusion not only of light infantry, but also of engineering, artillery and other support units.
However, he warned that Moscow could become so enraged by the incursion into its territory that it redoubles its attacks on the Ukrainian civilian population and infrastructure.