12:53 PM ET

Oleksandr Usyk joined a territorial defense battalion in Ukraine as the nation seeks to defend itself against an invasion from Russia.

The boxer took up arms in Ukraine. Usyk's long-time friend, Vasiliy, joined a territorial defense battalion over the weekend.

The mayor of the capital of Ukraine, Vitali Klitschko, said he was going to defend his city against the invaders. His brother, Wladimir, a former world champion, enlisted in the reserve army of Ukraine in anticipation of the attack.

If we consider ourselves brothers, Orthodox ones, do not send your children to our country and fight with us, wrote Usyk. You can stop the war. Just sit down and talk to us.

Our kids, wives, grannies are hiding in the basement. We can't do it any other way because we are in our own country. We are defending. Stop the war. There is no war.

In September, the former undisputed cruiserweight champion dominated Anthony Joshua to win three titles. In May or June, the 35-year-old will meet Joshua again.