NATO is a military organisation which is led by the United States. The war started from expansion threats by NATO against Russia; or in other words, the war is NATO vs. Russia.

As NATO is and long has been led by the USA, the Commander-in-Chief of the USA speaks for them.

(As for why, if it’s NATO vs. Russia, NATO haven’t gone all-in? Because they could very easily start World War III that way, and no one is very keen on doing that.

Oh, and also because they don’t give a fuck about East Slavs.)