Putin's opinion is that the eastern regions of Ukraine that have been controlled by Russian troops for years now and have separatists there vying for annexation

Putin's opinion is that the eastern regions of Ukraine that have been controlled by Russian troops for years now and have separatists there vying for annexation by Russia belong to Russia, not Ukraine (after all, they once did). Ukraine doesn't agree. It's also widely theorized (basically fact) that Putin has fomented revolution and supports separatist movements to gin up trouble and incite problems in those areas to further make his case. It's a land grab, and possibly a precursor to something much more. But only time will tell.