@Alistair
Annexation of a major european power may not be likely. Also quite possibly unnecessary. Suppose for a moment there is no NATO and the U.S. is not willing to use its “resources” on europe’s behalf. What does Europe’s existence look like within Russia’s unfettered sphere of influence?
I do not know the answer to that question. I am pretty certain it looks different than it does today though.
I'll bite. This situation assumes an extremely isolationist USA with little to no interest or exposure in Europe. It assumes a 'worst case' scenario where the USA is extremely reluctant to be involved at all, even in the face of serious Russian threat or political pressure.
Fairly soon after NATO is abolished, Russia will begin annexation of ex-soviet states. This will happen piecemeal under the guise of humanitarian or peacekeeping activities, but culminates with the integration of more of the Ukraine, Belarus, Estonia etc after 10-15 years. The EU will threaten economic sanctions, but without US support, these carry little weight. Worried, the major powers do, ironically, begin to meet and exceed the 2% GDP requirement for military spending, cutting by necessity social programmes and raising taxes to do so. This will be extremely unpopular, leading to social unrest and a rise of the left in many countries.
Eventually, maybe 30 years down the line, Russia will get uncomfortably close to the countries pulling the strings (threathening Poland perhaps, or Slovakia). Now seriously fearing for the cohesion of the EU, the major powers of Europe deliver a ultimatum. Cease and desist with these expansionist policies, or we will retailiate with a conventional or even nuclear response. Russia call the EU's bluff, knowing that politicians in the countries that matter for this type of response (Germany and France, primarily, the UK having Brexited years ago) haven't got the stomach or the resources for a boots on the ground war, nor will they risk the end of the world over
Slovakia of all places. Expansion continues. Having been shown to be unable to protect member states, the EU quickly disintegrates, with numerous small countries 'in the firing line' seeking to form closer alliances with the new Russian federation.
At this point, fearing the rise of a new, genuine Russian superpower, the US, and increasingly China is forced to become involved once more and the situation stabilises. Russia, hit with increasingly heavy sanctions from the US and China, which actually have the pull to make this work, becomes less aggressive, pulling back from Poland etc, but retaining much of their recently acquired territory in Latvia, Estonia, Belarus, the Ukraine. Having overstretched and seemingly 'surrendering' once more, the russian political climate changes, becoming more conciliatory, especially now they are trading from a position of greater strength with additional territory.
Russia focusses on cultural intergration of the annexed states, investing in infrastructure and education to advance their nations wealth and competitiveness in the global economy, something which is still pretty weak for the size of the country and the population. We see an increasing drive for foreign investment in Russia, which due to increasing concerns over the political ambitions of China, as well as the rising wages there, is welcomed by many developed countries. Russia becomes richer and interestingly the standard of living in many of the smaller Eastern European states improves significantly.
Meanwhile, the former EU states go back to the pre-EU state of relatively close trade links and shared investment of infrastructure, but without the increasingly closer political and military integration seen now. Peoples' trust in the 'EU project' is shaken and little appetite remains for an EU superstate. Military spending remains high, covered primarily by higher taxes, not at the expense of social reform.
Russia quitely begin to explore expansion to the South East, reasoning that the European and US powers will turn a blind eye as it doesn't affect them and may bring a bit more stability to the region...
Of course, there are a huge number of assumptions in this scenario, but this seems just about feasible as the worst case scenario.