In retrospect it seems obvious, but exactly how and why did Eastern European countries come to depend on Russian oil and natural gas after World War II?

In retrospect it seems obvious, but exactly how and why did Eastern European countries come to depend on Russian oil and natural gas after World War II?
Why did the Western Europeans reach energy accords with the Russians in the 1970s and early 1980s, building large-scale natural gas import infrastructures and increasing their dependence on Russian gas?