Why does the US treat the problems of Russia and Ukraine as its own?

In this question, I do not raise questions about the conflicts themselves. The question is different - how is the fact that the United States recognizes these problems as its own justified from a legal point of view?

Quote from today's executive order:

<...> the Russian Federation’s purported recognition of the so-called Donetsk People’s Republic (DNR) or Luhansk People’s Republic (LNR) regions of Ukraine contradicts Russia’s commitments under the Minsk agreements and further threatens the peace, stability, sovereignty, and territorial integrity of Ukraine, and thereby constitutes an unusual and extraordinary threat to the national security and foreign policy of the United States.

Why can't some South Africa just as well recognize the conflict between China and Taiwan as its own?