Russia recognised Ukraine's borders and its declaration of neutrality (July 16, 1990, with the adoption of the Declaration on State Sovereignty of Ukraine by the Verkhovna Rada), which was carried forward into Ukraine's Act of Declaration of Independence on August 24, 1991.
Since Ukraine starting fooling around with reverting that decision, Putin made it very clear what its response would be, were neutrality to be in jeopardy (notably Munich, 2007). When the Russian speakers were being shelled in the Donbas, Putin acted, after the fake Minsk agreements were dangled for many years to buy time. Hence the DPR and LPR.
Sevastopol has been the home of the Black Sea Fleet since 1783 (i.e. Catherine the Great) and if anyone thinks that Russia is willing to give that up, or is not able to maintain that, they are delusional and will be proven so.