LONDON — In the early 1980s, the Reagan administration warned its West European allies about their growing dependence on Russian gas. The Europeans paid no heed. They had found the Soviet Union to be a reliable supplier of gas over the previous decade, and they all — Margaret Thatcher included — refused to comply with a United States demand to stop their companies from selling technology with American origins to build new pipelines to bring more gas to Western Europe from Siberia.

Now, the Ukraine crisis has opened up a deep political rift with Russia, Europe’s largest single foreign supplier of not only gas, but also oil, coal and nuclear fuel — and that long-ago warning by the United States looks prescient.

For Europe, growing reliance on imported fossil fuels is a fact of life that will not change greatly for years. Europe’s domestic production of oil, gas and coal is in steady decline. Replacing fossil fuels with home-produced renewable energy like solar and wind power and building a new generation of nuclear reactors will be a long and expensive process.

The issue for Europe is not reducing overall import dependence, which is in any case a meaningless average for the European Union’s 28 states. Those nations have very different energy sources and patterns of consumption.