There are many alternative theories. I can mention two that I believe have some credence:
1. Liberal hegemony as described by Mearsheimer. Since the end of the Cold War, the US has been pursuing an unmitigated policy of targeted spreading of liberalism and democracy. That policy was mostly successful in Europe and select East Asian countries, but has been a disaster in the Middle East. Ukraine has been long ago targeted for regime change and liberalization, the culmination of which passed in 2014. According to this theory, EU countries most of whom are also strong believers in liberal democracy, essentially sleepwalked into disaster by ignoring the Realpolitk aspects of their foreign policy.
The irony of the tragic proliferation of liberal internationalism and democratic peace theory is that the key scholars behind neoliberalism were under no pretense regarding the structure of the international system. Core thinkers of the neoliberal school like Robert Keohane and Joseph Nye extensively borrowed the fundamentals of realist theory. In fact, the rise of neoliberalist thought in the 70s was very much a response to the emergence of neorealism and shared many realpolitik assumptions.
What we see today isn't a failure of neoliberalism, liberal instututionalism, or liberal democracy. Rather, since the end of the Cold War, what appeared to be the success of democratic thought became the dominant force for policymakers in the West. The resounding victory of the First World over the Second was attributed to political ideology, rather than political reality. And since the end of the Cold War, foreign policymakers (better known as the Blob) in the West have let that poisonous illusion cloud their judgement.
Make no mistake, it is highly possible the war in Ukraine today is the outcome that American policymakers wanted. Mearsheimer suggested that, if NATO continued to lead Ukraine down the primrose path, Ukraine would be ruined. But why should the Americans care? It's a good thing. They will fight Russia to the last Ukrainian. Mearsheimer's style of offensive realism won the Cold War for the Americans and they will continue ripping pages out of the playbook. War, not peace, is in America's interests.
Unfortunately for the Europeans, they are not nearly as cunning or deceptive as the Americans. If European leaders really did care about European interests, they would have done their best to avoid the war, knowing that Europe and Russia has long enjoyed a mutually-beneficial economic relationship. So you hit the nail on the head: The EU, being strong and blind believers in liberal democracy (the political ideology) sleepwalked into disaster by completely forgetting the fundamentals of international relations.
The only way for Europe to have an economic future is to first expel the toxic, ideology-driven policy inspired by American neoconservatives, and reverting back to the optimistic, rational, and mutually beneficial neoliberal ideas proposed in the 80s. And with that, Europe will hopefully remember the value of complex interdependence, the strength of pragmatic unity. And hopefully they might soon realise the stupidity and self-destructiveness of allowing Atlaticist ideology to infect their own decision-making.
Neorealism won the Cold War for the Americans, but liberal institutionalism won it for the Europeans. And if they want to survive into the 21st century, Europeans must never forget what it was that made them independent and successful, both politically and economically.
