Because it wasn’t a red line.

The whole Ukraine conflict goes back to 2013, when President Yanukovych was about to sign the EU Association Agreement which would be a stepping stone towards ascending to the EU. The far right Nazis hated Russia, obviously, so Ukraine joining the EU was a very big deal for them.

On the other hand, Russia (and Putin) had no problem with it, except for one part: Ukraine and Russia had tariff free agreement, and Ukraine signing the Association Agreement would complicate things.

Back in 2010, the Eurasian Customs Union was established and was composed of Russia, Belarus and Kazakhstan. It was an attempt to revive the Soviet era heavy industry chain in these post-Soviet states. It was also explicitly a customs union (as you can tell from the name), which means that it is a protectionist bloc that would help prevent foreign interference in the form of free trade arragements. Ukraine was also a country that was being courted by the Customs Union, because, well, it was a post-Soviet state and inherited some very advanced shipbuilding and aviation industrial capabilities.

The EU Association Agreement was a targeted attack against Russia’s Eurasian Customs Union. Unlike their Russian counterpart, the EU Association Agreement is explicitly a free trade agreement, which means that if Ukraine signed the agreement, it would not be allowed to join a customs union (protectionist bloc) in the future. Ukraine was forced into a dilemma where it could only choose one side: the EU or Russia.

This became a problem for Russia because Ukraine and Russia had existing tariff free agreement, and the free trade deal with the EU would allow European goods to enter Russian market without tariffs, but Russian goods cannot do the same against the European market. This was all made clear in Putin’s statements back in 2013, who explicitly said that this was an attempt to flood the Russian market with European goods to destroy its domestic industries.

However, Putin was not objecting to Ukraine joining the EU, he simply asked that the tariff issue be sorted and proposed a trilateral meeting between the EU, Ukraine and Russia to work out the specific details. The EU rejected Russia’s proposal, and pushed Yanukovych into committing to the deal. Again, forcing Ukraine to make a choice between the two.

Furthermore, the EU deal came with additional conditions: Ukraine would also have to commit to IMF loans and implement austerity (reduced spending on healthcare and education) as well as privatizing parts of their land (which had remained illegal in Ukraine even in the 2000s).

Around that time, Moscow also floated an alternative proposal that would have given Ukraine pretty much the same deal, but without the austerity part. Instead, Ukraine would actually get discounts on Russian energy.

President Yanukovych hesitated. Contrary to popular narrative, Yanukovych did not agree to a deal with Moscow. He had remained a pro-EU politician, and simply hesitated because the conditions were unfair to the Ukrainian people.

But that was already too much for the Banderites, who saw Yanukovych as betraying their cause, and set off the Maidan revolution by late 2013. Attacks and hostilities against ethnic Russians, who predominantly occupy the eastern regions of Ukraine (the Donbass and Crimea) ramped up.

On February 22nd, 2014, a coup was initiated and the far right nationalists took over the government. Yanukovych fled. The next day, on the February 23rd, the very first act of the coup regime was to repeal the Kivalov-Kolesnichenko Language Law, a 2012 law that granted official status to minority languages in the country, including the Russian language.

The act invoked strong condemnation from the ethnic Russians in the Donbass region, and seeing that an ethnic cleansing was about to take place, they rose up as a separatist movement against the Maidan coup regime. The Ukrainian civil war had erupted, and Russia - seeing that the ethnic Russians were about to be slaughtered - intervened. The rest is history.

For that, Russia would be subjected to an international sanction so deep that it would kill their economic growth for the next 4 years, and barely able to breathe when Covid hit in 2020.

Also, completely unrelated - the Nord Stream 1 pipeline, which guaranteed cheap natural gas supply to the EU, was commissioned in November 2011, just a little over two years before Maidan coup happened.

The problem with NATO came later after the Ukrainian fascists were defeated in the civil war, and paused with the Minsk Agreement. The fascists violated the truce fairly quickly, and were beaten again in 2015, and the Minsk II was forged, with German Chancellor Merkel and the French President Hollande giving their promises to Putin that Ukraine would take it seriously this time, for real.

Importantly, the Minsk Agreement would hand the Donbass back to the Ukraine authorities, under the condition that the local governments would be given the autonomy on cultural matters (such that Ukrainian politicians cannot simply ban the Russian language without the agreement of the local population). However, Russia would keep Crimea.

Ukraine never even bothered to adhere to the Minsk Agreement. Instead, over the next 7 years, they would be armed and trained by NATO, and reformed the Ukrainian Armed Forces to a formidable fighting force. Implicit (and not so implicit) in Ukraine’s actions showed that they intended to retake Donbass and Crimea by force. Putin continued to wait.

By 2019, President Zelensky was elected as a peace president with popular support from the ethnic Russian heavy regions (not part of the separatist state). However, when Biden took office in early 2021, a visit to the White House completely changed Zelensky’s outlook, and he went back on his promise to bridge the divide between ethnic Ukrainians and Russians. Instead, his rhetoric changed significantly and teased about joining NATO.

This raised the alarm on the Russian front, and resulted in the US-Russia Summit in June 2021, when the Russian side prepared hundreds of pages of proposal on the various scenarios for de-escalation. The Americans ignored Russian concern, and in August 2021, and then again in December 2021, the US sent Javelins and Stingers to the Ukrainian side, further flaming the tension.

By early February 2022, the Ukrainian military had been mobilized, and artillery shelling into the separatist territory ramped up by an order of magnitude. Russia made its move.

Also, another yet completely unrelated incident - Nord Stream 2 pipeline completed its construction in 2021, and was awaiting commission at the time the war broke out.

Probably because if one were to invade Russia (and how Russia was actually invaded in the past), it would be through Ukraine. Attacking Russia through Finland is a logistical nightmare, you just couldn’t supply a big offensive force through there. The Baltic states are a tiny countries, easily dealt with; can’t put an army there that’s large enough not to be overrun by Russia.

Ukraine is large, flat, the borderline is massive, the population of Ukraine is substantial. Turning Ukraine into bulwark against Russia, allied with NATO, is massively worse than all the other border countries. You can see how much trouble Russia has in this war dealing with just the Ukrainian army, imagine if there was a multi-million NATO force fighting there.

Geography, the Baltic-Russian border is heavily forested and hilly and in the case of Estonia there’s two giant lakes that make up 80% of the border

Meanwhile, the wide open plains of eastern Ukraine north of Donbass lead right into the heart of Russia

in addition to location, not like russia could do shit in the 90s about baltics + persistent denial of nato entry to russia despite them twerking for usa