At the very least they don't have the tools to make nuclear hay anytime soon. That's true of most kinds of political intelligence. To have any political significance, intelligence of any kind must be promptly and properly used. The aforementioned George McClellan, for instance, the night before the battle of Antietam, had the very best possible intelligence, not from a spy, but from Confederate misfortune. He had, in fact, a copy of General Robert E. Lee's movement orders for the following day.
History is full of spy stories, many ironic but very few with any lasting impact on their subjects.
Mata Hari, for example, despite her reputation as a spy in World War I, did very little, if any, damage to the French. Historians still argue over the significance of the Alger Hiss case and the Julius and Ethel Rosenberg case. Even the greatest intelligence coup of World War II did not always have the battlefield significance one would have expected. But they were often unable to use these secrets, because doing so would have revealed the even more secret fact that they were reading the German high command's secret orders. Finally, it's good to remember that very little of our military technology is truly secret.