Primary vs. Secondary Sources in Military History

A bundle of wartime letters found in a closet is a primary source. A regimental history written fifty years later is a secondary source. Most of the time the difference is that simple. Military history, though, is full of in-between cases: memoirs written decades later, official histories built from interviews, oral histories recorded in a veteran's eighties. Knowing how historians sort their evidence will help you read any war story with a clear, fair eye.

The basic definitions

The Library of Congress describes primary sources as the raw materials of history: original documents and objects created during the time being studied. Secondary sources are accounts that retell, analyze or interpret those events, usually from a distance in time or place.

Many university library guides add a third group, tertiary sources. These are encyclopedias, textbooks, indexes and other reference works that summarize or organize what others have written. They are good for background and for pointing you toward better evidence, but researchers rarely build an argument on them.

A simple way to remember the difference: a primary source was there, or was made by someone who was there. A secondary source was made by someone looking back and trying to explain.

What primary sources look like in military history

Armies generate enormous amounts of paper, and soldiers generate their own. Common primary sources include:

• Letters and diaries written by service members and their families during the war.
• Unit records, such as after action reports, daily unit journals, general orders, staff reports and unit histories. The National Archives notes that U.S. Army unit records from the Second World War are mainly found in Record Group 407, the records of the Adjutant General's Office, and are generally used in person at the National Archives at College Park, Maryland, rather than online.
• Individual service records, which document where a person served and when. Our guide to finding military service records explains how families request them.
• Photographs, maps, orders and overlays made at the time.
• Newspapers and broadcasts from the period, which show what people at home were told.
• Objects: uniforms, equipment, medals, ration cards, even a soldier's pocket Bible with notes in the margins.

Each of these carries the fingerprints of its moment. A unit journal records what the staff believed was happening that morning, before anyone knew how the battle would end. That is exactly what makes it valuable.

What secondary sources do well

A rifleman sees a few hundred yards of a battle. A clerk typing the after action report sees the paperwork that reaches headquarters. Neither sees the whole picture. Secondary sources exist to put the pieces together.

A good example is the U.S. Army's official history of the Second World War, a multivolume series produced by the U.S. Army Center of Military History and widely known as the "Green Books." Army historians wrote these volumes after the war, working from the wartime records and interviews the Army had gathered. The full series is now available online from the Center. For most readers, it is a secondary source: careful, detailed, but written by people looking back.

Secondary sources let you see causes and consequences, compare one battle with another, and weigh many witnesses against each other. They also carry their authors' choices about what to include and what to leave out. That is why historians read the footnotes. A strong secondary source tells you exactly which primary sources it rests on, so you can check them.

It depends on the question you ask

Here is the point that surprises many students: the same item can be primary or secondary depending on what you are trying to learn. One university library guide uses Theodore Roosevelt's book The Naval War of 1812 as an example. If you are studying naval battles of 1812, the book is a secondary source, since Roosevelt wrote it long afterward. If you are studying Roosevelt himself, it becomes a primary source, because it shows how he thought as a young man.

Military history offers many similar cases. S.L.A. Marshall's 1947 book Men Against Fire made famous claims about how many American riflemen fired their weapons in combat during the Second World War. For a question about combat in 1944, it is a secondary work whose evidence has been sharply questioned. For a question about how the postwar Army thought about training, it is a primary source, because it shaped that very debate. Our article on how historians weigh eyewitness accounts walks through that case in detail.

Memoirs and oral histories: the in-between cases

A memoir written by someone who served is firsthand testimony, so historians usually treat it as a primary source. But it was written after the fact, sometimes long after. By then the author may have read other books about the war, attended reunions, and spent years thinking about what it all meant. None of that makes a memoir less honest. It means the memoir is evidence of two things at once: what happened, and how one person came to understand what happened.

Oral history interviews, such as those collected by the Library of Congress Veterans History Project, work the same way. They capture voices and details that no official report would ever record. Our article on memory and the war memoir explains, with respect, why recollections naturally change over a lifetime.

Neither kind is automatically right

It is tempting to think that the closer a source is to the event, the more accurate it must be. Closeness is valuable, but it is not a guarantee.

• An after action report may be written hours after a fight by an exhausted officer who had only partial information.
• A letter home may leave out danger so a mother will not worry. Silence in a letter does not mean nothing happened.
• An order shows what a commander intended, not necessarily what took place.
• A newspaper story from 1944 may reflect wartime censorship as much as events on the ground.

Secondary sources have their own weak spots. An error in one popular book can be repeated by later writers who never check the original record. Sometimes a single striking statistic travels for decades before anyone asks where it came from.

The historian's habit is not to trust one kind of source over another, but to set them side by side. When a soldier's diary, a unit journal and an enemy report all describe the same river crossing on the same morning, confidence rises. When they disagree, the disagreement itself becomes a clue worth following.

Questions to ask of any source

The Library of Congress encourages teachers to approach a primary source in three steps: observe it closely, reflect on what it suggests, and then ask questions about who made it, why and for whom. Those steps work for family papers as well as archival records. In practice, the questions sound like this:

1. Who made it? What was their rank, role and vantage point? Could they actually see what they describe?
2. When was it made? During the events, days later, or decades later?
3. Why was it made, and for whom? A report to a commander, a letter to a sweetheart and a book for the public each have different purposes.
4. What does it leave out? Every source has gaps. Some are accidental and some are deliberate.
5. What else agrees or disagrees with it? Look for independent sources that describe the same moment.

Why the distinction matters to families

Many people come to military history because of one person: a grandfather who rarely spoke about the war, an aunt who served as a nurse, a parent with a box of medals and photographs. Understanding sources helps in two ways.

First, it tells you where to look. Unit records and service files can confirm dates and places. Secondary histories can explain what a unit was doing and why. A family member's own letters or recorded interview add the human dimension that neither of the others can supply.

Second, it helps you hold different accounts together without forcing them to match perfectly. A veteran's story, an official report and a historian's summary are not rivals. They are different kinds of evidence, each answering a different question. Read side by side, they give a fuller and fairer picture than any one of them could alone.

If you are ready to start gathering your own primary sources, our guide to recording a veteran's story and the free printable interview guide are good next steps.