With publication of his book Declassified: 50 Top-Secret Documents that Changed History, historian and author Thomas B. Allen opens the curtain on what amounts to an alternate view of some of the most famous and decisive military conflicts in history. From Elizabethan England to the Iraq War, here are the ploys, the devices, the deadly games, the crucial secrets that explain what really happened and why, as well as what the leaders knew and when they knew it.

What are the earliest known instances of military espionage?

In the Bible [Numbers 13] are references to spies involving Moses: He sends a representative from each of the tribes to Canaan. He wants an intelligence report with a variety of responses. In this case he gets 12.

Who was the first spymaster known to history?

Sir Francis Walsingham worked for Queen Elizabeth I and is the first spymaster who really runs a network. He had spies in the court of the king of Spain, court of the king of France, the Vatican. He got merchants—who could easily travel—to feed him information. And he was a cryptographer. He worked out some pretty good code systems.

Did other governments learn about espionage from Walsingham?

There isn’t anything like him until the American Revolution, where you find military intelligence on both sides. With his discovery that Dr. Benjamin Church was a British spy, George Washington wakes up to the idea that he’s not just going to run an army, he’s going to do military intelligence.

How did Washington come to be regarded as “the father of American intelligence”?

With Major Benjamin Tallmadge, he set up a network primarily in New York, and they work with a codebook. It consists of three digits that refer to a word or to a person. Washington is 711. Washington comes up with all of this. The proof is in his papers; he has long, detailed instructions on the use of invisible ink. He says to write an ordinary letter and then write between the lines using invisible ink. Washington—I don’t know where all this comes from. It’s marvelous.

What kinds of intelligence would Patriot spies have pursued?

They were looking for solid military information. How many cannons? How many men? Count the tents.

Did Washington do counterintelligence?

Yes. A guy comes to him volunteering to be a spy. Turns out he was a British spy, and Washington turns him and runs him as a double agent. There are things like that all through the Revolution. Washington learned that Church’s mistress had delivered a letter that was supposed to go to a British officer but wound up in Patriot hands. It’s a bunch of numbers and letters, so Washington gives it to two bright guys he knows. He tells each one to crack it. They do, and both come up with the same solution. It was very wily of Washington to have two people do it.

Did Washington’s successors build on his espionage efforts?

No, it all goes away. But we still don’t know the full extent of Washington’s activities and probably never will.

When does the U.S. resume active military intelligence?

The Civil War. The Confederates set up a secret service. The Union never really set up a formal military intelligence operation. They hire what they call “detectives,” and one of them is Allan Pinkerton. Under an assumed name, he goes on the staff of George McClellan. He provides pretty bad intelligence, though he does crack one attempt to assassinate Abraham Lincoln when Lincoln was on his way to his inauguration. But there’s no real military intelligence apparatus in the Union; it’s just every Union general for himself.

When did the U.S. military create a formal an intelligence unit?

Around the time of the Philippine insurrection, an officer says: “We don’t really have any intelligence. We’re fighting these guerrillas, and we don’t know a thing about them.” He develops an intelligence apparatus in the Philippines. But when we enter World War I, we still don’t have one. Of course, the French, British and Germans all have incredible intelligence operations. So in World War I we start something called the Military Intelligence Department.

Before we had computers, how did people crack codes?

It was people just grinding away, looking for patterns. This may be apocryphal, but when the British were working on German codes in World War II, on April—whatever Adolf Hitler’s birthday is—everybody in Germany sends greetings to the Führer. “Happy Birthday, Führer!” British cryptographers noticed that on Hitler’s birthday a lot of coded messages looked very similar, and that got them started.

Has all the important intelligence from World War II been declassified?

The big area that still could hold a few secrets is the relationship between the Soviet Union and the United States on a military level. What did they know about our plans? What did we know about their plans?

How did deception factor in during the Normandy invasion?

The big thing was the feint. We convinced the Germans that Normandy was a feint, and the real invasion was going to come at Calais. It pinned down a whole German army. Hitler believed in Calais. He also believed there was going to be an invasion of Norway.

Was it plausible the Allies would have invaded Norway?

The British get a handful of people to make believe they are an army, using all kinds of radio techniques. Their double agents send information back to Germany about the army in Scotland that’s planning to invade Norway. The British send engineers into Sweden to look at the railroad tracks into Norway. The Swedes get upset because they think the British are going to come into Sweden and invade Norway by rail. And word of all this gets to the German ambassador of Sweden, and he sends that information back. Everyone goes all aflutter in the German general staff, and they keep troops in Norway.

What was the British “Double Cross System”?

From the beginning of World War II the Germans tried to get agents into Britain, by parachute, by submarines and smuggled in from Ireland. Every German agent was caught almost immediately. They were given the choice: Either work for us, or we’ll hang you. About a dozen chose not to work for the British, and they were secretly hanged. The British then ran these agents, but the Germans thought the agents were still their own guys.

How did code trickery enable the U.S. Navy victory at Midway?

We were reading the Japanese naval code, and intelligence officers at Pearl Harbor had determined there was an invasion plan afoot by a Japanese fleet of four aircraft carriers and accompanying ships. The Japanese used two-letter designators for places and had said, “We will rendezvous for the invasion of AE.” Well, this could have been the Aleutians, Hawaii or Midway. So one of our intelligence officers says to Midway, “Send an open message, or a message using a low-grade code that the Japanese can crack, that your water situation has broken down and you need water.” Then they monitored Japanese radio, and sure enough the Japanese sent a message about needing water for the invasion of AE. That’s how we caught them. The Navy was able to figure out exactly where the Japanese carriers were—and sink them.

What surprising Cold War revelations have surfaced since the collapse of the Soviet Union?

Cables have surfaced on the Soviet side that show unequivocally that the Soviets pushed the North Koreans into invading South Korea with the assurance that the United States would not bother anyone about it. That was Stalin’s theory. The other big story was that a lot of guys who were in the OSS in World War II and stayed in intelligence when the OSS morphed into the CIA were actually Soviet spies. We really had quite an infiltration in our government and intelligence agencies. That was a tightly held secret.


Originally published in the October 2008 issue of Military History. To subscribe, click here