As the leaders of the Group of Seven countries meet in Germany this week, there are rising calls to end the diplomatic impasse with Russia over Ukraine. But wishful thinking is the worst basis for policy making.

That was true in 1967, when Pierre Harmel, Belgium’s foreign minister at the time, issued a report advocating the type of diplomatic engagement with the Soviet Union that would eventually become détente. The report came at a difficult time for the Western alliance. France had withdrawn from NATO’s military command...