As Ukraine tries to contain a pro-Russian insurgency convulsing its eastern region, a perhaps more significant struggle for the country hinges on what happens beneath the ground here in a placid woodland in the far west, on the border with Slovakia. This is where about $20 billion worth of Russian natural gas flows each year through huge underground pipelines to enter Europe after a nearly 3,000-mile journey from Siberia. It is also, the pro-European government in Kiev believes, where Ukraine has a chance to finally break free from the grip of Gazprom, Russia’s state-controlled energy behemoth.