Ist die Kernlandtheorie aus dem Jahr 1904 im Russo-Ukranischen Krieg immer noch sinnvoll?

It appears to be useful history and not least a perspective from the early 20th century. Whether or not the theory holds water, for Russia it isn’t as though they can alter the geographic or economic situation. They are where they are and if they seek relevance or security (or any one of a number of conditions), they have little choice but to confront their Western and South Western borders.

The Russian central south is mountainous and dry and broadly inhospitable until it reaches the Indian subcontinent. They’ve played the Great Game in Afghanistan, Iran, etc to know pretty much what they’re facing there.

The Russian east and south east is Mongolia (broadly not supportive of large population) and China, another great civilization.

It is no big surprise that the Baltic states, Poland, etc rushed to join NATO and the EU. Belarus and Ukraine are the border countries that have to live the reality that any resurgence of Russia requires them to be subject to Russian rule simply because Russia has no meaningful alternative.

A Russian defeat is necessary for Europe. Anything else is nonsensical pandering for the time being. Putin has made it clear that his Russia cannot be satisfied by the status quo and any alternative to that status quo involves Russia holding on to, at a minimum, the border countries by force. Putin is clear that trade and global order are, at best, secondary considerations for Russia.