Consider the hypothetical, Texas has decided to secede and has had a majority of the population vote on a referendum to do so. Rather than becoming independent they wish to join Mexico, Mexico agrees as there is a large portion of Texas that speaks Spanish and there is a historical precedent of Mexico owning that territory.

The rest of the United States doesn't want this and considers this illegal, a nationalist division of the U.S military decides to engage in sustained artillery bombardment of the cities: Dallas, Austin, and Houston.

Mexico seeing the devastation of people now considered Mexican organizes two separate talks to try and work out some sort of cease fire and pathway forward that respects the will of the people. The nationalist division pays no mind to any sort of agreement and continues the bombardment, Mexico decides that enough is enough and sends its military in to secure Texas as well as destroy the nationalist division.

In this analogy, the United States is Ukraine, Texas is the Donbass, and Mexico is Russia.

Sadly war is hell, if only the peace talks weren't sabotaged by Boris Johnson otherwise the Russia-Ukraine war would be over.