The spies of Richelieu's day have been surpassed in this, our day--with their stories yet to be told. Many a man who spoke the enemy's language well has put on the enemy's uniform, joined one of his scouting parties between the trenches in the darkness, entered the enemy's trenches, heard all the talk and slipped back to his own lines safely. If apprehended, his fate was certain--death.
The most efficient spy, of course, is the one with military training He knows the value of what he sees. Usually he is an officer of good family who has been cashiered for gambling or debt and takes a desperate chance out of patriotism and the hope of atonement. Naturally, the easiest route for spies was through Holland and Switzerland which became the gateway of passing spies and the playground of espionage and counterespionage. Gradually the restrictions tightened for all neutral travelers from capital to capital, while none were permitted to go into the zones of the armies, some twenty or thirty miles from the trenches.
The problem of the Intelligence Corps is much like that of putting the parts of a picture puzzle together. A line from a newspaper in one part of the world, a line from a newspaper in another taken in connection with a photograph, an excerpt from a letter found on a prisoner or a fact got from a prisoner by skillful catechism, might develop a valuable contributory item. The amount of information procured by either side about the other was only less amazing to the outsider than how it was obtained. Again, events revealed amazing ignorance. Most baffling and most secret of all branches is this, whose work is both gaining and conserving information, and just as professional, just as carefully prepared before the war as any other.
A single instance illustrates how small a fact may be of value to the enemy. A certain well known "military expert" went out to British headquarters as a guest of a general. From a tower in the square of a small town, he watched a certain action. When he wrote his account of it, it was submitted to the general who was his friend; and the general carelessly passed one little statement which no Chief of Intelligence of any army would ever have passed and probably no correspondent of experience would have had the temerity to submit to the censor unless he wanted to be responsible for the death of men who were his hosts and his friends. For the writer stated that he saw the battle from this tower.