NATO has stabilized Eastern Europe because the overwhelming might of the West was enough to guarantee reasonable security. Without NATO Eastern European states would have two choices, Finlandization and balancing Russia.
Promises and perils of Finlandization
Finlandization got its name after policies Finland adopted during the Cold War. Finland remained neutral and avoided confronting Russia. In return Finland was allowed to remain a capitalist democracy and to engage in lucrative business with Russia. Finlandization is sometimes the approach offered for countries like Estonia.
The problem with Finlandization is that it was probably a unique situation that is hard to replicate. Soviet Union avoided pushing Finland too hard because Finland had fought fiercely during the Winter War so the Soviets had a lot of respect for Finnish threat of military resistance if they were pushed too far. Finland used both political concessions and military deterrent to achieve a certain way of living next to Russia.
After the Second World War Czechoslovakia also tried to maintain some degree of freedom like Finland. It didn't work because communist influence was so strong that in 1948 they just organized a coup to cement their power. When Czechoslovakia tried again in 1968 to become a bit more like Finland in its relations with the USSR, Brezhnev sent in tanks. Czechoslovakia just didn't have Finnish military capacity so Russians felt free to push Czechoslovakia to ever greater concessions.
The same is probably the true with all post-Soviet states. None of them has in the eyes of Moscow a status similar to Finland because they haven't fought their own Winter War against Moscow and thus Russia feels that it can push ever further, until she meets some resistance. An advice to finlandize would probably mean for post-Soviet states an advice to accept ever-greater Russian meddling.
Balancing Russia
The reason why Baltic states like Estonia would be unlikely to finlandize would be decisions taken in Poland and Ukraine. Those nations would quite surely try to balance Russia, thus offering the Baltic states an alternative alliance for protection.
These days both Poland and Ukraine are staunchly anti-Russian. End of Western support would not encourage them to find an agreement with Russia, but would instead encourage a full confrontation with Moscow. Never in centuries has geopolitical situation been more beneficial to balancing the power of Russia, even without NATO. Germany is no longer a threat to Poland. Ukraine's turn to the West in 2014 was a major geopolitical event. Ukraine could have easily fallen for Moscow but instead pro-Western government maintained control over about 90% of pre-war territory and population. Now the anti-Russian alliance of Poland and Ukraine has about 60% or Russian population and about 45% of Russian GDP (in 2015, measured in US dollars). This looks like a solid Russian superiority but the Polish-Ukrainian alliance has to worry only about Russia, while Moscow has several security issues: Syria, Caucasus, China, Ukraine, even the Arctic. Russian power is split, it had four different navies and an expensive nuclear program.
This future Polish-Ukrainian alliance already has a name – Intermarium. This means land between the seas, namely between the Baltic and the Black Sea. The history of Intermarium stretches back to time after the First World War. Back then it didn't work out because too many nations were to be involved in it and Ukraine lost its independence after only a few years. It's different now. The core of the alliance has only two members – Poland and Ukraine, who only have one main problem – Russia. This core Intermarium would be strengthened by cooperation with other nations like Romania or Sweden.
Geography is here the key. For the first time in 500 years Poland is no longer a direct neighbor with Russia. Between Poland and Russia are Ukraine, Belarus and Lithuania. The Russian enclave of Kaliningrad is cut off from mainland Russia as long as Lithuania remains independent. This makes Lithuanian independence very important for Poland and gives Lithuania a secure place in the Intermarium alliance. Latvian independence helps to secure Lithuania and to cut Russia off the Baltic Sea, so Latvia can also count on Polish support, but less. Estonia is even further in the north than Latvia and is thus less vital for Polish security. This means Estonia has to work even harder to achieve protection of the Intermarium.
Domination of the Baltic Sea
Poland would inevitably try to neutralize the Russian fleet on the Baltic Sea. Russian fleet's main base in Baltiysk in Kaliningrad is within range of Polish artillery so the Poles can consider this problem to be under control. The second port of the Russian navy is in Kronstadt in the Gulf of Finland. Before WW2 Estonia and Finland had plans to block the gulf so that Soviet navy couldn't get out of the gulf. It didn't work back then because in 1939 Estonia decided not to fight. It didn't turn out well for Estonia back then so now Estonia has decided to fight for her freedom.
There are currently no known plans to block the Gulf of Finland. NATO doesn't need to do it, it's strong enough to defeat the Russian navy in the Baltic Sea, thus keeping the sea open to supply the Baltic states. Intermarium on the other hand would find blocking the Gulf of Finland the easiest way of keeping Russian navy out of the Baltic Sea.
Doing it would force Estonia to spend heavily on the navy. Estonia would need to develop mine laying capacity and would also probably need land- or shipbased anti-ship missiles. Developing those capacities will drive Estonian defense spending way above the current level of 2% of GDP. Also, all Baltic states would need to strenghten their anti-aircraft capacities and land forces. This will easily drive defense spending in all three states to 3% of GDP or even higher.
Destabilyzing Russia
That's not the end. For balancing Russia Intermarium would probably try to weaken Russia by supporting anti-Russian forces. This would for instance mean giving anti-aircraft MANPADS, like the Grom system, to opposition forces in Syria. West has so far avoided it, because there is a danger terrorists could use it against civilian planes, but Intermarium is not going to worry too much about a few planeloads of dead German turists. War in Ukraine has already killed thousands and the events between 1930-1950 killed millions of Ukrainians and Poles. This hangs heavily on decisionmakers in Warsaw and Kyiv.
Intermarium would also arm and finance anti-Russian forces in Caucasus and would try to establish a contact with Chechnya's strongman Ramzan Kadyrov who has a large private army. Intermarium would also be tempted to support Taleban in Afghanistan in the hope that Taleban's win will destabilyze the whole Central Asia, thus creating another nightmare for Russia. And Intermarium would support every Chinese attempt to capture Russian lands east of the Urals.
Despite Russian claims NATO hasn't tried to destabilize and divide Russia, mostly out of fear of what might happen to Russian nuclear weapons in case of Russian collapse. Intermarium on the other hand would be glad to see Russian collapse because it would diminish Russian threat to them. The weak and the strong behave differently. NATO is so strong that it can think about the longe term dangers of Russian instability. The weak can't afford such luxury because if they get their policy wrong the weak may have no "long term" to worry about. Memories about loss of independence are very vivid in Intermarium. Countries of Intermarium will anyway think that after Russian collapse nuclear weapons will remain in the hands of state actors and even if terrorists get them, those nukes are unlikely to be used against Intermarium countries.
For the Baltic states costs of Intermarium may not be limited to higher defense spending. Intermarium countries at the Baltic Sea are much richer then those on the Black Sea. Russia on the other hand is applying most pressure on the poorer countries on the Black Sea. This requires financial support of the richer Poland and Baltic states to poorer Ukraine and maybe Georgia to build up their infrastructure and militaries. In fact, the richer countries of Intermarium would need to copy some of the policies of EU that they have so benefited from. Those costs would also be significant. For Estonia both increase of defense spending and support for Ukraine may need an additional 2% of GDP. If there would be a Russian arms buildup near the Baltic states of if Russia would seriously attack Ukraine those extra costs may become even greater.
Going nuclear
Intermarium may achieve a conventional deterrent against Russia but will remain vulnerable to Russian nuclear blackmail. The only way out is getting an independent nuclear deterrent. Ukraine has some know-how and Poland has money so their nuclear path could be quite fast. To speed things up they might team up with other nuclear countries like North Korea, Iran or Pakistan.
Even a few nuclear warheads might give Intermarium an effective nuclear deterrent. They are of course very far from a credible second strike capacity but Ukraine is so close to Russia that Intermarium would not need nuclear submarines for deterrent. Several large Russian cities, including Moscow, are so close to Ukraine that Intermarium could target them with nuclear warheads delivered either in short-range missiles or fighter jets. Even a small probablity of losing a major city to a nuclear strike will make Moscow think twice about its nuclear threats and that's already something of a deterrent.
For the Baltic states all this would be just another terrible dilemma. Baltic states are way too small for any nuclear deterrent so they would prefer a Finnish policy of fighting a serious conventional war while ignoring Russian nukes. Poland and Ukraine would think of nuclear weapons mostly because those weapons are available to them. Baltic states would need to decide how they would react to Polish and Ukrainian nuclear weapons because being allied with them would make them targets of Russian nukes. Would Estonia be willing to have Polish nukes here so Poland could target St Petersburg? Even switching sides won't help because if the Baltic states would be allied with Russia then Russian bases in the Baltics would be tempting targets for Polish nukes.
How to avoid this bleak scenario?
If the West would withdraw from Eastern Europe at least some developments discussed in this article are likely to take place. No Western pressure on Poland and Ukraine is likely to stop those developments because pressure would be seen just as another indication that the West is an unreliable partner and that thus Intermarium needs an independent deterrent against Russia.
Continuation of current policies of NATO and EU would be great, because they help to solve Eastern Europe's security issues without the destabilizing effects of balancing Russia. If US would drop out of NATO there still are several things Europe can do. Europeans can strenghten the European pillar of NATO, they can keep EU operational and the sanctions against Russia in place.
EU sanctions against Russia are here an important milestone. If EU would drop the sanctions against Russia without serious improvement of the situation in Ukraine Eastern European nations like Poland could feel like they now have to respond with measures of their own, like direct military support to Ukraine with lethal weapons. If that would take place then it will be a sign that my warning of an independent Eastern European security policy would be becoming true.
