Ally, Member or Partner? NATO’s Long Dilemma Over Ukraine.

NATO promised Ukraine full membership in 2008, but without explaining how or when. Putin sees that promise as an ongoing threat to Russia.

BRUSSELS — Ukraine presents NATO with a dilemma many years in the making — one the alliance, itself, helped create.

In 2008, NATO — an American-led alliance explicitly created to counter the Soviet Union — promised membership to two former Soviet republics, Ukraine and Georgia, but without specifying when or how.

Russia saw the offer as a potential threat on its borders and an encroachment into the heart of its sphere of influence, the most serious in a series of affronts and humiliations by the West since the fall of the Soviet Union. From the outset, some NATO nations questioned whether the offer of membership was a wise move, and it is not clear that the promise will ever be kept, but predictably, it has fed a lasting conflict with President Vladimir V. Putin.