STORY: Russian tanks, troops, and trucks rolled through Moscow streets this week, rehearsing for the nation's annual Victory Day parade, commemorating the Red Army's defeat of Nazi Germany in the Second World War.

But the celebration this year comes as Russian forces are once again engaged in bitter combat.

Russian invaded Ukraine in February in what President Vladimir Putin calls a "special military operation."

Tenacious Ukrainian resistance beat back Russian attacks on the capital, Kyiv, forcing Moscow to retreat and recalibrate its aims.

Now, as Victory Day approaches on May 9, and some wonder what kind of victory Putin will present to his people.

"There is absolutely no doubt whatsoever that in the Kremlin they didn't anticipate that the fighting, the war, would still continue on the 9th May."

Kataryna Wolczuk is an expert on Russian politics at Chatham House.

She says that if Putin can't claim a major prize, he may settle for a smaller one: finally seizing control of the battered and besieged city of Mariupol.

"So there has been a lot of speculation about what the Kremlin would like celebrations to look like and the fighting in Ukraine, but especially in Mariupol, has been turned into this almost mini, small scale, symbolic victory. So Mariupol is very important."

Ukrainian fighters have so far denied Russia full control of the port city. Block-by-block fighting reduced much of Mariupol to rubble.

A small number of soldiers and civilians are now holed up the Azovstal steel works, and Russian fighters and Russian-backed separatists have been unable to dislodge or defeat them.

"There were a lot of hopes that Putin and the people in the Kremlin just could proclaim a victory and somehow de-escalate. Now we know that the 9th of May is not going to be the date, Russia will continue fighting whatever happens in Mariupol, despite the casualties on the Ukrainian and on the Russian sides. "

Putin is set to speak at the May 9 victory parade before a massive display of Russian military might.

In rehearsal, a squadron of Russian warplanes this week streaked over Moscow in a "Z" formation, a symbol associated with the Russian military offensive in Ukraine.

The Russian president has repeatedly compared the current campaign with the effort to defeat German fascism 80 years ago.

And the May 9th parade will commemorate a war in which Soviet Union pushed the Nazis all the way back to Berlin.

But this year, Putin may have to settle for the cinders of a single Ukrainian city.