Hall of Fame boxers Wladimir and Vitali Klitschko are preparing for the biggest fight of their lives -- the Ukrainian brothers are vowing to defend their country from the Russian attacks.
Wladimir spoke out about the invasion of his country on Thursday ... condemning Putin's actions and pledging to join in the fight with the Ukrainian people against the Russian regime.
"I am writing to you from Kyiv, the capital of a country at war, a country being attacked and invaded from all sides," the former heavyweight champ said. "It is not 'the war of Ukraine,' it is Putin's war."
"Putin wants to call into question the geopolitical balance across the whole of Europe, he dreams of being the defender of the Slavic peoples wherever they live, and he wants to restore a fallen empire whose demise he has never accepted."
Wladimir went on to say the Ukrainian people have chosen democracy, but he knows democracy is a fragile regime and him and his brother, Vitali, are ready to fight for it.
Vitali -- the mayor of Kyiv -- told "Good Morning Britain" in an interview he was "prepared to take up arms" if need be to halt the Russian invasion.
The Klitschko brothers also released a passionate video ... calling on all countries to observe what's happening and unite against Russia.
Arnold Schwarzenegger -- a longtime friend of the Klitschko bros. -- sent a message of gratitude to the boxing legends ... lauding them for their courage in the midst of this difficult time.
"Vitali and Wladimir @Klitschko, I am thinking of you, my friends. You were my heroes in the ring and you're my heroes now."