A large part of Europe, especially Eastern, Northern, Central, and South-Central Europe including the Balkans, has been dependent on Russian natural gas since the Soviet times. Little changed with the breakup of the Soviet Union as Russia was no longer perceived as a strategic adversary. Moreover, Russia's dependence on gas sales in Europe as a source of important revenue for the state-owned Gazprom was considered a guarantee against the use of gas supply as a strategic tool by Moscow. However, a dependence on the Russian energy supply resulting from a system of pipelines built back during the Cold War has increasingly proved to be a disrupting, negative factor in European energy security. This dependence already has negatively manifested itself several times throughout the region and has a potential to massively disrupt European military, economic, and humanitarian situations if a substantial and/or prolonged interruption of the gas supply occurs.