The main claim for a western audience by Russia is that the “eastward expansion” of NATO is threatening Russia. Let’s dig into the claim and what is behind it.

Summary

While NATO was founded to fight against an eastern bloc, it was repurposed after the fall of the Soviet Union. NATO is not a single entity that expands its territory, it is a coalition of sovereign states that assure each other mutual security guarantees and to which all (European) states can apply to.
It was never, to no time of it’s existence, there to be against Russia.

Russia, a state that excels at lies and intrigues today and mastered hybrid warfare, accuses NATO of breaking a promise of no eastward expansion. While opinions differ, there maybe was a promise, orally and given to the Soviet Union (hint: the Soviet Union doesn’t exist anymore since over 30 years). The world, including NATO, changed a lot in the few years after that. Russia (now we’re talking about Russia, not the Soviet Union) on the other hand signed (not orally) years later (1994/1995, so after the alleged promise) a treaty to guarantee Ukraine’s independence and to refrain from any military attack on it (Budapest Security Memorandum).

Let’s take the argument seriously, for once, and assuming that the promise is true. Russia, as well as Ukraine etc were part of the Soviet Union (SU): so the SU is complaining that parts of the SU joined NATO, and NATO promised to the SU not to expand. So the SU now needed to attack the SU because the SU joining NATO would be a threat to the SU since then the SU could help NATO to attack the SU.

Nothing says that – even if that promise were to exist and had any value – this promise is obsolete like the fact that parts of the SU already joined NATO. The world has changed, Russia is not the Soviet Union.

NATO

NATO came into live after the Second World War in direct response to the Soviet Union as a military threat which on the other hand founded the Warsaw Pact in order to counter NATO. This lead to the cold war era view of a world split into two blocs.

Post Soviet

After the fall of the Soviet Union, the mission of NATO changed as the initial threat was no longer existing and therefore NATO was not needed anymore either. Since a large organizational structure grew over time, instead of dissolving, NATO was repurposed to constitutes a system of collective security.

The main mission of NATO is stated on their website as “NATO’s purpose is to guarantee the freedom and security of its members through political and military means.” It is an alliance of sovereign, mostly democratic states. The main purpose is Article 5 of collective defense in case a member state is attacked. While this makes it foremost a defensive alliance, NATO has also intervened in conflicts as it started close cooperation with the UN as a peacekeeping force, bringing a new purpose to the organization.

NATO’s eastward expansion

Wikipedia offers a good factual summary.

As NATO became a purely collective defence organization and wasn’t anymore an “east vs west” bloc, a necessary step was to open up the membership to other (European) countries and allow all of them – in principle – to join. Denying the possibility to apply for a membership to any country would undermine the legitimacy of the organizations new purpose and a claim to be an “anti-eastern” organization would be righteous.

Soon after the fall of the Soviet Union and the Warsaw Pact, eastern countries started to apply to NATO, which were granted full membership rights after several years and once conditions were fulfilled. This made NATO “expand to the east”.

This expression – NATO’s eastward expansion – is often found in news and especially in claims on the side of Russia targeted towards western audience. And yet, it already shows a bias, it directly follows the Russian narrative that NATO is a single entity, a superpower, that is expanding its “power” to the east. As seen before, NATO is a coalition of states that accepts new members if they actually apply (and fulfil conditions) because they want to join. Or to put it in the words of former NATO ambassador Garcevic “It wasn’t for the West or Moscow to decide whether those countries should join NATO. Each and every country has the right to decide its alliance and affiliation”.

So the real question is, why did so many eastern countries apply to NATO? Especially given that Russia, which emerged after the fall of the Soviet Union, with a few other countries founded the CSTO, a security organization consisting of six member states.

Why did eastern countries join?

After the fall of the Soviet Union and the dissolution of the Warsaw pact, many countries leaving the bloc joined NATO over the years.

It’s important to state that they did not join because NATO threatened them in any way (otherwise, e.g. Serbia that has been bombed by NATO would have joined long time ago). Rather, they claim to be afraid of being (again) invaded by Russia as Russia started a couple of wars with its neighbours.

This claim is, especially nowadays, substantially confirmed, by nothing else than Russia’s second (the first was in 2014, Crimea and Donbas) invasion of Ukraine. Or the Russian invasion of Georgia. Or the threats to take “serious military consequences” against Sweden (which isn’t even bordering Russia) and Finland in case they would consider joining NATO.
In fact, it is highly likely that the membership in NATO of the baltic states prevented Russia to invade them or to meddle with them as done with Georgia or Ukraine.

Russia, as much as it sounds biased, is best described as a sort of “bully” that meddles with its neighbouring states. Fear and suppression were the tools in the Soviet Union to control occupied countries and they remain the same in the current Ukraine invasion with civilians being deliberately targeted. No one better confirms this view than countries that have actually been under Russia, which are currently the ones who offer the most support to Ukraine. For a good reason.

It is no wonder that states seek protection of their sovereignty.

On the other hand, there has not been a NATO country that left for the CSTO.

Claim of violation of an agreement

It is often claimed by Russia that NATO promised not to expand eastwards and/or broke an agreement.

While some sources hint at statements made by NATO in this direction, neither is there any agreement nor is there any serious indication for such a claim, especially not formal. PolitFact, discussion on StackExchange, friendsofeurope

One thing that is often interpreted wrongly is that NATO promised verbally and in an agreement for Germany’s reunification in 1990 that no NATO troops would be stationed in east Germany. Sometimes, the formulation of “no inch further east” is found, but this needs to be put into context: during this time, eastern Nations were still in the Warsaw Pact, which only dissolved in 1991. In the time of the German reunification, where such statements were made, a possible eastern expansion beyond Germany was not considered a realistic option by anyone.

This “promise” was made orally to the Soviet Union (!). A state that ceased existing over 30 years ago.

In short: it was never written in any agreement and it was with a state that does not exist anymore. Namely that it either was never agreed upon (or just about east Germany) or that it was seen as too unrealistic at that time and there were no discussions about it.

Russia and NATO

Wikipedia provides a good summary of the NATO-Russia relationship.

With NATO’s new purpose and consequent opening to all European states, it was also possible for Russia to join NATO. As absurd as this may sounds, this was considered a real possibility and was often spoken about. Some sources claim that Putin himself expected to be invited and get a special process for its application in order “not to wait in line with other unimportant countries”. It is also claimed that NATO demanded more steps towards democratization and the respect of human rights from Russia.

Either way, (AFAIK) Russia never formally applied to NATO and NATO never rejected Russia.

The relationship of NATO and Russia improved a lot however by signing the Founding Act on Mutual Relations, Cooperation and Security providing a roadmap for cooperation of NATO and Russia. In general, Russia and NATO were not opposed around the 2000s, in the end, there is no reason. The Soviet Union collapsed, the spread of Communism isn’t any “danger” from Russia.

In the end, why Russia never joined nato may be attributed to a few factors such as Russia asking for special treatment in the application process and NATO asking for more democracy. Whatever it was – and this is probably attributed to both sides – Russia never applied to NATO and was therefore also never rejected.

Is NATO a threat to Russia?

Yes and no. And this is where the controversy and misinterpretation may comes in as it is what Russia (in it’s empire narrative) likes to tell. Russia is not the Soviet Union (!), as often implied in arguments.
TL;DR: No. Russia’s security is not at all at threat by NATO.
Russia’s possibility to interfere in sovereign states, yes.

No: militarily, NATO is absolutely not a threat that it would ever attack Russia.

It’s post cold-war

TL;DR: NATO’s purpose was never to fight against Russia. There is no reason nowadays to fight Russia.

During the cold war, NATO had a clear purpose to counter the threat posed by the Soviet Union while the Warsaw pact was to counter the threat of NATO.

But this period is over. The east is not a threat anymore, including Russia. There is no fear of attacks or the “spread of Communism”. Russia, for the West, is simply of “no interest”. Why invade? Occupy Moscow? For what? You can do trade to get resources. And the cost of an invasion would be way too high anyways, Russia is huge and has nuclear warheads… never!

But Russians don’t wanna hear that. It’s a way of belittling Russia who once was this empire.

Still, the West doesn’t have an interest. No matter how much Russia wants to paint this picture.

NATO is a defensive alliance

TL;DR: NATO is mainly defensive to protect its members. It never expanded its territory militarily.

Let’s assume for the moment, all other things aside, that NATO would want to station weapons at Russia’s border with Ukraine or to attack from Ukrainian soil: this has nothing to do with NATO but depends solely on Ukraine, whether it is a NATO member or not.

1. NATO members still need (in peace time) to approve troop movements through their country.
2. To move troops through a country, that country doesn’t need to be in NATO, but only needs to allow it.

The main thing that comes with a NATO membership is article 5 which declares an attack on one country to be an attack on all countries. This is what Russia fears, because it means it cannot attack Ukraine anymore.

Being integrated into NATO makes many of the processes of moving troops around easier. But it is by far not a free pass nor is it a requirement to move troops through a country. As a reminder, Ukraine wants to be in the EU and NATO and therefore would – most likely – also without a membership gladly allow NATO weapons to be stationed on its soil.

Nuclear weapons

TL;DR: Nuclear weapons make forces and distances nearly irrelevant. A single one going off is too much.

Russia is still a large nuclear power. In case of any military attack, it could launch nuclear weapons. And the problem with nuclear weapons is the uncertainty: No matter how good any defensive counter measure is to intercept an attack, it needs to counter 100% of all attacks and no less. If even a single nuclear warhead would detonate, say in Paris? Warsaw? New York? It would be an unbearable cost.
This certainty will never be given as long as not all capabilities of Russia are exactly known. Furthermore, no missile interception system has a known rate of 100%.

Even with Ukraine joining and increasing the capability of interception of missiles, there are still submarines in the oceans that can carry nuclear warheads and strike from any direction and the vast majority of Russian borders.

The probability of a single nuclear warhead slipping through any defense is simply non-zero and it won’t be in any foreseeable future. The possible cost is simply too high for any western country as they are in a stable, wealthy state.

NATO is already close

TL;DR: NATO has already a very close border through Latvia. Ukraine would barely change anything.

… and has done not a single provocation.

Russia often claims to be surrounded by NATO and that is a danger. A quick look at a world map reveals that “encircled” is maybe wrong. But there is a bit of truth as most of Russia is in the West and Ukraine is about 600 km from Moscow away.

So is Latvia, which is already NATO. Estonia is about 150 km from St. Petersburg away. NATO already is in close proximity to Russia and could use it to launch missiles.

It also shows however NATOs behavior: no missiles or troop build ups have been done since this countries joined in the beginning of the 2000s.

If another country, namely Ukraine, joined, this would not change too much. It helps a bit having a second corridor, but actually just minimally. And since NATO’s article 10 clearly states that only European Nations are to be admitted, it’s not that Russia would be “encircled” any time soon.

Yes. Because of Ukraine’s sovereignty

Yes: by guaranteeing Ukraine to be a sovereign state (a status Russia explicitly denies). This doesn’t affect Russia directly, but indirectly.

Russia likes to be surrounded by unstable states and doesn’t like democratic, stable states. The reason is simple: stable countries are harder to temper with (this is not per se exclusive to Russia). Putting pressure on politicians, using puppet governments, provoking an uprising… all of this works way better if there is no stability.

Russia tried this on Ukraine before with Yanukovych as he was pressured (around 2014) to not sign a deal with the EU which staged protests and ultimately lead to the Maidan protest and the following removal of Yanukovych from office.

Shortly after Russia started meddling again: Russia occupied Crimea and helped pro-Russian militia (Russian and Ukrainians) and with regular Russian military to start occupying parts in Donbas. It was through a strong support from the population and volunteer fighters that not more regions than the Donbas were occupied.

This events have clearly shown Ukraine that Russia cannot be trusted and the public opinion to join NATO raised from a clear minority in 2013 (~20% pro to 60% against) to a clear majority (~60% vs 30%) in 2015 (source).