By Deborah Haynes, Security and Defense Editor

More precise information on the number of Ukrainian soldiers in Russia remains unclear, as commanders are deliberately keeping quiet about the secretly planned mission.

But there are likely to be thousands, and elements from at least three well-equipped brigades will be on site, using tanks, armored vehicles, artillery guns and drones.

It is also difficult to measure how far the Ukrainian attackers have advanced. According to Russian military bloggers, they have advanced to about 19 kilometers from the Ukrainian border.

Videos allegedly showing Ukrainian soldiers raising the yellow and blue flag of Ukraine over Russian territory, including in the town of Sudzha and a settlement near the Ukrainian border in the neighboring Belgorod region, have been widely circulated on social media.

Under pressure, Russia immediately sent reinforcements and released footage of its military countermeasures. But the Ukrainian offensive is now in its sixth day and fighting is still ongoing.

Analysts commented on the events by saying that this was the first invasion of Russia since Adolf Hitler in 1941.

But Ukraine’s attack is not the act of an aggressive power seeking to seize territory.

Rather, it is the counterintuitive action of a country that was invaded by Vladimir Putin’s Russia a decade ago – with the seizure of Crimea and parts of eastern Ukraine – and subsequently further devastated by Moscow’s full-scale invasion in February 2022.

Kyiv’s counter-invasion in Kursk is only the latest – albeit probably boldest – attempt by Ukraine to push Russian forces back from its own territory.

To counter Russia’s far greater military strength, Ukrainian commanders had to be innovative from the start and be willing to take great risks.

Two and a half years ago, the numerically and technically inferior Ukrainian troops – supported by Western weapons – managed, against all odds, to prevent Russia from taking Kiev.

They then forced the Russian invaders to retreat from all of northern Ukraine.

Months later, in September 2022, the Ukrainian side introduced further innovations.

They launched a surprise counteroffensive against Russian troops occupying the northeast of the country – just as Russia was facing a simultaneous but far stronger counterattack in the south.

In this advance, Ukraine recaptured large parts of the territory in the Kharkiv region. Its forces also continued the counterattack in the south and recaptured the southern city of Kherson.

However, Russian positions in other parts of southern Ukraine and in the Donbass to the east are far more entrenched and difficult to conquer.

With Ukraine’s Western allies supplying increasingly lethal weapons such as tanks and long-range missiles, expectations of a second Ukrainian counteroffensive grew last year.

But these efforts failed because the combat units were unable to break through the heavily fortified Russian positions.

This time, Kyiv, under the leadership of Colonel-General Oleksandr Syrskyi, Commander-in-Chief of the Ukrainian Armed Forces, has taken a different risk.

Instead of preparing his troops for another attempt to storm Russian positions in Ukraine, politicians have chosen to send their soldiers to Russia, where the border is – perhaps surprisingly – far worse defended.

Ukraine is very unlikely to have the ability or desire to take and hold much, if any, territory, but it has dealt a humiliating blow to Mr Putin and brought Russia’s bloody war in Ukraine a step closer to the Russian people and their president.