Originated from the cowboy's call sign in western movies, this is what the cavalry shouted when they charged. Americans like to watch westerns, and if they watch too many, they imitate them. Especially in the army, they are closer to cowboys, just like a slogan encouraging them not to be afraid of death. Americans regard cowboys as a symbol of adventure and courage, and like their tone, clothes and revolvers, which can be regarded as cultural heritage.

It's like the Russian army's name is Ula. Wula means long live in Russia and also comes from its own culture.