"Please call to the attention of the President of Mexico that the employment of ruthless submarine warfare now promises to compel England to make peace in a few months."
"Zimmermann."
Alfred Zimmermann was the German Foreign Minister.
The German defense to this piece of absolute proof was what we have since learned to expect from Germans;—
"We were not doing it. And anyway, it was not unfriendly, and we had a perfect right to do it."
The once great German machine was now without its leaders, and all it could do was to carry on a number of small local agitations, with no directing intelligence. A very few months after the publication of the Zimmermann letter, the United States itself went into the war. Then the constant struggle between detectives and enemy-aliens became even more serious. A new problem faced the Secret Service and its co-workers. That was to keep the German spies over here from sending to Germany information that would be of value to her in a military way. No knowledge of the movements of troops, of fleets, or of supplies must be allowed to leave America. At all costs the war plans must be kept secret.
The spies tried to send information to Germany by many different ways, such as by cable to Denmark, Switzerland, or any other neutral European nation, and then by telegraph into Germany; or by telegraph to Mexico, and then by wireless to Germany; or by wireless to a neutral ship on the ocean, which would relay to Germany by her wireless. The first and most important thing for the spy in every case was to get his message out of this country.
To prevent this, the United States established censorships. There were telegraph censors, watching the wires into Mexico; there were postal censors, examining the mails; but the most interesting was the cable censor, who had to keep all the cables free from enemy use. Although cable censorship was done by the Navy Department, its work very often overlapped that of the Secret Service. Here is a typical example of how these two worked together, not correct in details but accurately showing the method followed in a great many cases:—
In June, 1917, some of General Pershing's first troops sailed from New York, in number about 15,000 men, in 13 transports. On that very day a Spanish firm in the city filed a cable to Spain, saying:—
"Quote 13 millers at 15 per cent."