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In reality, the German proposal was daft. To be sure, the United States was weak militarily, but Mexico, emerging from years of bloody revolution, was in no condition to challenge its stronger neighbor. No matter. The Wilson White House released the telegram to the Associated Press, and headlines blazed news of the “threat.”

Historians for the most part assert that the Zimmermann telegram drove Wilson’s decision to enter the war a few days later. But Mr. Boghardt did an intense analysis of the editorial position of leading U.S. papers and found that few changed their stances toward the war because of the telegram. The key precipitant toward war was the German sinking of a British liner, killing two U.S. women, and the announcement of unrestricted submarine warfare.

Nonetheless, Mr. Boghardt concludes, “The United States certainly would have entered World War I regardless of the telegram, but by removing Wilson’s final doubts about the wisdom of joining the Allies, it accelerated U.S. intervention, though perhaps only by a few weeks.”

Mr. Boghardt’s work is a masterpiece of intelligence writing. By following the hard evidence rather than relying on historical assumptions, he provides an incisive case study on how intelligence can affect national affairs.

Joseph C. Goulden, formerly Washington bureau chief of the Philadelphia Inquirer, is the author of 18 books.