This claim comes from political scientist John Mearsheimer, who shares his views in a YouTube video. He shares that Putin does not resemble 1940s Hitler and is not attacking Ukraine in an attempt to conquer/act as a hegemon. Rather, he says that Ukraine's becoming a member of NATO and the resultant expansion of NATO along Russian borders is the reason for the war. In his video, Mearsheimer states that "The possibility of Ukraine becoming a member of NATO is simply unacceptable to the Russians. It is an existential threat" (03:33 - 03:43). https://bsky.app/profile/jerry44939.bsky.social/post/3lzvvcioujc2z
Upon searching Mearsheimer's claim, I found only a few other sources or figures supporting this claim. In contrast, several news sources and NATO itself disagreed with this claim, providing evidence to support their views. Overall, there is no crystal clear answer; there are claims made by Putin that support this claim, but there are also many pieces of evidence that contradict Putin and this claim. As I will explain below, there are sources that explain how NATO expansion caused Putin to feel threatened, quoting Mr. Putin himself. The conclusion that war was waged due to NATO as a "threat" comes from Putin himself. In contrast, other sources say Putin's claims of feeling threatened are historically illogical and misleading. Essentially, Putin may believe what he says, that Ukraine was a real threat and the war is a result of that, or it may be political propaganda and a convenient excuse to cover the intentions of Russian domination. Evidence is given by various sources that show how Putin's claims are misleading, that they do not follow, and that they are worthy of a close look. All in all, it is somewhat up to the reader's discretion to come to his or her own conclusion based on the facts and statements given. Based on what I found, I would mark this claim as false because while NATO may be part of the reason for the fuel in this fire, it does not reasonably seem to be exclusively about NATO.
First, in support of this claim, a New York Times article explained how, since the 1990s, NATO has taken in most of the Eastern European nations in the Communist sphere. Putin has described this Soviet disintegration as "one of the greatest catastrophes of the 20th century," robbing Russia of its "rightful place" among the world's greatest powers. He has called NATO's expansion "menacing" and the "prospect of Ukraine joining it as a major threat." In this article, there is the theme that Putin's war was a response to Ukraine's pursuit of NATO membership. This is based on some of Putin's stated opinions. https://www.nytimes.com/article/russia-ukraine-nato-europe.html
To understand this issue fully, it is important to understand the main reasoning behind supporters of this claim. After the Cold War in 1990, Putin claimed the U.S. Secretary of State James Baker and other Western leaders assured the Soviet Union that, beyond Germany, NATO would not expand any further eastward. While this idea was thrown around in post-Cold War negotiations and assertions, it never became part of any treaty or formal agreement; it is published nowhere in writing. Nonetheless, Putin blames the West for "betraying" Russia, as much of Eastern Europe has joined NATO, and uses this reasoning to fuel his aggression. Thus, we can see that although Putin may feel as if he was betrayed or threatened, it was not based on any legitimacy or facts of history. There was no violation of any agreement, and NATO has consistently asserted that any country's decision to join is a nonaggressive action and one that is entirely its own. This information can be found on several news sources, but was pulled directly from the American Foreign Service Journal, which exclusively represents the U.S. Foreign Service. https://afsa.org/did-nato-expansion-really-cause-putins-invasion
Continuing down the NATO conversation, I found it quite helpful to read their response posted on their official website https://www.nato.int/cps/en/natohq/115204.htm. NATO directly asserts that it did not promise Russia it would not enlarge after the Cold War. Particularly, NATO's founding treaty "includes a clear provision that opens NATO's doors to 'any other European state...'" So, there was no intentional, deliberate "eastward expansion" of NATO; it has simply enlarged in response to the desires of countries that wish to join. NATO gives further evidence to support the facts that NATO's deployments are not threats to Russia, and NATO is not encircling Russia. This contradicts one of Putin's main concerns, that he felt threatened by NATO's "expansion" and encroachment onto Russian borders. NATO shares that "only 11% of Russia's land border is shared with NATO countries."
To further support this claim that NATO presents no threat to Russian borders, and has not in the past, is shown when considering the response from Russia when two border countries, Finland and Sweden, ended decades of neutrality with Russia and joined NATO. When this occurred in 2022, Putin declared that this was "no problem." This information comes from the Atlantic Council, a notably non-partisan outlet https://www.atlanticcouncil.org/blogs/ukrainealert/putin-uses-nato-as-an-excuse-for-his-war-against-ukrainian-statehood/
Finally, I think readers can see the inconsistency here. From NATO's evidence, the reality that no such agreement of NATO limitations exists, and the non-response from Putin in the past to NATO developments, it does not seem terribly convincing that this war was waged because NATO suddenly became an "existential threat" to Russia. Perhaps Putin somewhat feels this way, and perhaps part of the reason for this war is due to Putin's distaste for NATO. But that does not mean it is the full explanation for the roots of this war; the Ukraine war is likely not all about NATO expansion.
