I have read 3 Books by Peter Zeihan. In 2016 book Absent Superpower he said Russia will invade Ukraine in early 2020s. It sounded like fiction back then, until Russia indeed invaded Ukraine in 2022.
According to 2016 book by Peter Zeihan the reason behind Russia invading Ukraine is that Russia has a border security problem. It has a long border, topographically in flat terrain, and it is very difficult and expensive to defend Russian borders and this makes Russia vulnerable to outside powers. The problem of defense of Russian borders have 3 solutions:
1) Predeploy a 3-4 mln man army near the dangerous areas of the border, which is not a possibility due to Russia's economic and demographic constraints
2) Create a small, highly mobile force that can be quickly deployed to any area of Russian border if Russia faces an attack - that is not a possibility due to poor road and railway infrastructure of Russia and long distances, which make deployment and supply of forces difficult
3) Expand borders by invading neighboring countries and create a buffer zone and establish the border along the geographic barriers like mountains of the Caucasus or Dnepr River for example - the buffer zone and these defensive geographic barriers will make defense of Russia easier and cheaper.
According to Peter Zeihan 2016 book, demographic degradation of Russia, forces it to solve the border security problem through invasion of neighboring countries as quickly as possible. Today Russia has soldiers and engineers, but in 2030 it can be too late to invade because Russia will have much less soldiers and engineers due to demographic processes.
In 2016 Zeihan wrote that Russia will invade Ukraine in early 2020s and conquer it easily in three weeks and then it will invade Moldova, Georgia and the Baltic States. USA will not fight a war with Russia, but will supply weapons to these countries. Turkey will remain neutral, while Sweden and Finland will attack Russia after it invades the Baltics. Sweden will destroy Russia's Baltic Sea fleet and impose a naval blockade to St. Petersburg.
In 2016 it sounded like fiction, but in reality Putin indeed planned invasion of Moldova (Transnistria) after Ukraine, as the map of Russian plans shown on Belarussian TV in February 2022 showed. And South Ossetia in Georgia planned a referendum on joining Russia in July 2022. Turkey remained neutral, while Sweden and Finland entered NATO and started a militarisation program.