Does anyone really know why the United States went to war against Germany in World War I? Anyone? If you say the sinking of the Lusitania, that has some bearing but is not the final reason. It was something called the Zimmerman Telegram. The German Foreign Minister sent a telegram to the German ambassador in Mexico. This telegram informed the ambassador to meet with the Mexican President to inform him: a.) Germany is about to restart un-restricted submarine warfare, b.) Germany would be willing to support Mexico in its bid to take back Arizona, New Mexico, and Texas, and finally c.) open talks with Japan to become part of this alliance against the United States. British intelligence intercepted and decoded this message. It was presented to President Wilson on Feb. 24th, 1917 and then released to the American public on March 1, 1917 and it was on April 6th, 1917 that the United States declared war on the Central Powers. Looking at the original Zimmerman Telegram I can see something else that probably infuriated the American populace, the encrypted telegram went through the United States [Galveston, TX] on its way to Mexico.
"No other single cryptanalysis has had such enormous consequences." - David Kahn, author of The Codebreakers.
A more well known signals intercept has to be the U.S. War Department sitting on pins and needles waiting for the 14th part of Japan's newest diplomatic effort to avert war with the United States on the evening of December 6th and morning of December 7th, 1941. Alas the 14th part was anti-climatic as all it did was temporarily break off negotiations with the United States and with the accidental late delivery by the Japanese ambassadors of the whole document, it just became another example of Japanese perfidy in not negotiating in good faith.
I lack enough information on the current temptest to make any valid judgement, but as an amateur historian I feel I must point out the value of intercepting and decoding foreign telecommunications. The feeling I get is the President signed off on running down this one captured terrorist's contacts as quickly as possible before his capture leaked out rendering the information useless in the hope of rounding up more enemy combatants before they caused further death&destruction.
U.S. Archives, Zimmerman Telegram.
European History, Zimmerman Telegram.