Brother war celebration

Why are these people dancing jubilantly and waving the flags of warring Russia and Ukraine?

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Is the apparent joyous war celebration in the video above related to European “brothers” killing each other, as in World Wars I and II?

Without context, the story here seems unclear.

Ukrainian president Zelensky previously considered Russians and Ukrainians to be ethnic and cultural “brothers.”

In the video below, he says:

Russia and Ukraine are brotherly people. …

We are one color, one blood. We understand each other, irrespective of language.

Why do our leaders have us fight wars that don’t serve American interests?

Visibly elated to see Russians killed, US Speaker of the House Nancy Pelosi says:

[T]here is a school of thought that thinks the anti-aircraft missiles and the rest are a very important way [to supply Ukraine with weapons].

Myself, when I see those 40 miles of [Russian] tanks, I'd like to take out those tanks [and thereby kill tens of thousands of young Russian soldiers, destroying their families’ future].

Video below at 3:30:

How is the group that leads Americans and Europeans to spend their blood and treasure in killing “brothers” related to the group that privately celebrates these wars?