army had so many secrets to
guard; never has it required such complicated measures of protection
against espionage. In Napoleonic times, it was enough to know that
your adversary was marching a hundred thousand men along parallel
roads. This your cavalry scouts might discover; or a spy who had
crossed the frontier in an unfrequented place might be watching the
enemy's army and counting his numbers as they passed. Now the
frontier is an intact line of trenches.

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