The collapse of oil prices—added to the weight of Western financial sanctions—has hammered Russia’s economy. But Moscow is also suffering from the fall in prices of another important energy export: natural gas.

Gas represented 14% of Russia’s export revenues in 2013, compared with 54% for crude oil and oil products. But its gas is more important to Europe and Ukraine.

Gas is more than a commodity for Moscow: It has been a political tool that has helped it assert its influence in Central and Eastern Europe and beyond. Because of pipeline architecture, some European Union countries have had few alternatives to buying gas from Russia.

But its grip over Europe’s gas markets is weakening. As natural-gas prices have fallen in the European market—less dramatically than oil but still by at least a quarter—Moscow’s revenues have tumbled. In the longer term, further losses in Russia’s pricing power and market share in Europe appear inevitable.

Russia has itself largely to blame. The Soviet Union treated gas as a commercial commodity and deliveries were unaffected by tensions between East and West. Under President Vladimir Putin, gas exports became a political instrument, and Russian disputes with Ukraine caused stoppages in supplies in 2006 and 2009 to Ukraine and to EU countries to its west.