Explainer: Why is Russia's Putin so focused on Ukraine?

Russian President Vladimir Putin answers questions about his article titled "On the historical unity of Russians and Ukrainians" in Saint Petersburg, Russia July 13, 2021. Sputnik/Alexei Nikolskyi/Kremlin via REUTERS

(Reuters) - Ukraine has become the main flashpoint in Russia's relations with the West after a build-up of tens of thousands of Russian troops near its border and a series of tough statements from President Vladimir Putin setting out his "red lines".

Here are three sets of reasons why Putin feels so strongly about Ukraine and has chosen to bring the crisis to a head.