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For more than a decade, the United States has nurtured a secret intelligence partnership with Ukraine that is now critical for both countries in countering Russia.

You have a preview view of this article while we are checking your access. When we have confirmed access, the full article content will load.A Ukrainian Army soldier in a forest near Russian lines this month. A C.I.A.-supported network of spy bases has been constructed in the past eight years that includes 12 secret...