By attacking and occupying them, the way it always does. Which is why those neighbors wanted to join NATO in the first place.
And what an idiotic thing it was to grant NATO mutual defense treaty membership to 12 new countries in Central and Eastern Europe! It was already a stretch to pledge America to total warfare for the sake of Germany and other Western European nations, but did we really intend to stake our own survival on Romania, Bulgaria or Estonia? Back in 1999 and 2004 many thought the world would remain static and the USA would always be invincible, but did anyone understand the stakes of such blank-check promises? Thank goodness NATO was thwarted in Georgia and Ukraine, or else we would be committed to a war for the Black Sea that we could not possibly win today.
And speaking of blank checks, the USA has now written far too many of them over the past seven decades -- far more than we can now cash. Like an overextended bank that fears a run by its depositors, our greatest threat is that our numerous far-flung enemies might strike at once. Emboldened by our increasing weakness and angered by our continued meddling, a hostile world is waking up to the realization that the United States cannot keep all of its promises at the same time.