2009 Russia–Ukraine gas dispute
In 2009, Russian natural gas company Gazprom refused to conclude a supply contract unless Ukrainian gas company Naftogaz paid its accumulating debts for previous gas supplies. The dispute began in the closing weeks of 2008 with a series of failed negotiations, and on January 1, 2009 Russia cut off gas supplies to Ukraine.
Source: Wikipedia — 2009 Russia–Ukraine gas dispute (CC BY-SA 4.0)