There is no evidence of Russian ambition into Europe. So that would be a starting point. Then, I’d look at the peace that have been maintained on the continent for more or less 80 years. Established and maintained with free trade. Russia was happily selling oil and gas - and buying real estate and professional service in Europe. Germany believed in it so much, they built a crucial pipeline to seal the deal - to the annoyance of the USA.
In the issue of the Ukraine. Most people imagine the Ukraine as a united territory with a clear, defended border. This has never been the case. Eastern Ukraine has never been very loyal or obedient to the elites in western Ukraine. Eastern UKraine is Russian speaking , and has always overwhelmingly supported Russian leaning presidents - because if the deep cultural and economic ties that exist.
None of this is to diminish Russian aggression and illegal invasion of Ukrainian territory. However, it speaks to a localised dispute that everyone should surely work to diffuse, and not escalate.
In light of this, and in light of the mutual defence treatise and agreement that exist is Europe, and the fact the Britain and France have nuclear weapons - my answer is yes, Europe is sufficient defend.
I would further suggest, and you may not like it, that should Europe disentangle from the US (which is being discussed but i believe impossible ) - Europe would be even less of a target .
Europe should be working harder a peace, not at war, and I suggest that the boys of the internet calling for war, would feel very differently when the letter telling them to report the local military base arrived at their house.
In the first case, it’s easy to see with hindsight that Hitler should not have appeared nor trusted . But context is important: Europe had lost 20 million people in WW1. It is entirely understandable that they should pursue peace and not war.
In the second, there are two sides to an agreement, and Ukraine did not honour its undertakings to Russia.
Switzerland obviously has no treaties of mutual defence, they are not in nato, nor in the EU, so run a much larger army that countries that have them.