Putin is a megalomaniac.
Russia does have some security concerns. Their only warm water port for their navy that gives them access to the Mediterranean is in Crimea.
They also have most of their natural gas lines that feed Europe pass through Ukraine. This is where Russia's wealth comes from. So I guess one may be able to understand why Putin would want to have a puppet president in Ukraine that he could control and not a president whose loyalty is with the West.
Putin is still a thug though...and there have been many comparisons to his actions and some of the events that lead to WWI.
Dan
