PolitiFact, March 2022
So far, the military conflict following Russia’s invasion of Ukraine has remained within the borders of Ukraine. But it may not stay there—and if not, there’s a chance of a military faceoff between Russia and the U.S.-led North Atlantic Treaty Organization, or NATO.
Ukraine is bordered by four NATO members: Hungary, Poland, Romania and Slovakia. (A fifth neighboring country, Moldova, is not a NATO member.) Further north, Russia borders the three Baltic states, each of which belongs to NATO: Estonia, Latvia and Lithuania.
Because Ukraine is not a NATO member, the alliance has no formal obligation to intervene to stop Russia’s invasion. However, if Russia attacks any of the NATO-member nations, it could prompt NATO to invoke what’s known as Article 5, a collective-defense mechanism that NATO itself describes as “the very heart of NATO’s founding treaty.”