On June 15, the Espionage Act quietly turned 100. The birthday went unacknowledged. Perhaps that’s understandable: To the extent Americans think about the Espionage Act, they think of it as a relic of World War I, when it was used to censor critics of the war. But the act is still significant — indeed, maybe more significant than when it was introduced in 1917. That’s because, as a tool of classification rather than censorship, it allows the federal government to wage its contemporary “war on whistleblowers.”

The evolution of the Espionage Act has done much to shape the freedom of political debate in the modern U.S. What began as a vague statute that gave President Woodrow Wilson power to jail critics and censor the news media during World War I has transformed into a powerful law that regulates political discourse by policing secrets, not speech. Censorship did not end when the war did. It evolved.

Despite its name, the Espionage Act has always been a broad statute. When it was passed in 1917, it prohibited classic spying. But it also banned interference with the draft and, after amendment in 1918, any criticism of the war effort.

In its earliest drafts, the act actually went further. The first draft gave the president power to ban the circulation of “any information relating to the public defense.” This limit on freedom of speech aroused critics who, even during wartime, argued that such powers were unconstitutional. As a result, Congress struck this provision from the bill.

Yet the Espionage Act remained powerful enough to pave the way for the modern regime of state secrecy. These powers were forged from the broad and vague clauses within the act that were left open for interpretation. Notably, the act prohibited circulating information “related to the national defense” to those without authorization. But it didn’t specify what, exactly, constituted information “related to the national defense.” That’s because the sections of the act that outlined the process for defining defense information and security authorization — and which had given the president broad powers over that process — had been cut in 1917 to protect press freedom.

The resulting ambiguity was left to the courts to decipher during the interwar years. Two cases in particular revealed the deep tensions between free speech rights and the protection of security information. In 1938, the Justice Department prosecuted two men, Mikhail Gorin and Hafis Salich, for transferring naval intelligence reports to the Soviet Union. And in 1941, it prosecuted Edmund Heine, a German automotive executive, for transferring information about the American air industry to Volkswagen in Nazi Germany.

In both cases the defendants claimed that they did not engage in “espionage” because the information they had transferred was publicly available. Gorin’s naval intelligence reports included no technical information, just lists of the observable activities of Japanese Americans and their fishing vessels. Heine’s accounts of American air power were based on magazine clippings and visits to air shows. How could such accessible information count as “national defense information,” let alone “espionage?”

The Supreme Court upheld the prosecution of Gorin and Salich in 1941, asserting that naval intelligence reports clearly fell within common understandings of “national defense” information. But just four years later, the Second Circuit Court of Appeals overturned Heine’s conviction. The opinion, written by famed Justice Learned Hand, concluded that information had to be kept “secret” if it was to qualify as “national defense” information for the purposes of the Espionage Act.

In other words, if the government made no effort to conceal the information, then its circulation could not be a threat to national defense. As the Justice Department explained in a failed appeal of Hand’s decision, such an interpretation of the Espionage Act meant that the state would simply need to keep more secrets.

At the same time, a small and obscure group within the wartime administrative apparatus, the Security Advisory Board, took the statute’s application into its own hands and wrote new regulations to expand and solidify the protection of state secrets.

The result: President Harry Truman’s 1951 Executive Order 10290, which finally resolved the ambiguities that haunted the Espionage Act since Congress first amended it in 1917. The order established the modern classification regime, defining uniform categories of secrets and standardizing practices for handling that information. Most importantly, it defined classified information as “affecting the national defense of the United States within the meaning of the espionage laws.”

Ironically, 34 years after Congress refused to grant the president sweeping authority to regulate the circulation of defense information, the president successfully claimed that authority by executive order. Yet there has been little pushback, and the classification system has never been seriously challenged by free speech activists. Unlike the original draft of the Espionage Act, Truman’s order censored only those government employees who leaked information, not the press that published such material.

By regulating the disclosure of information rather than its publication, the modern secrecy regime seeks to balance the interest in national security with the First Amendment.

Despite these attempts to safeguard freedom of the press, the impact on the freedom of public debate is stark. The media cannot publish what it does not have; government employees risk criminal prosecution if they share unauthorized information with the news media. Because about 80 million documents are classified each year, the Espionage Act has become a powerful tool blocking the flow of information to the public.

A desire to protect the freedom of the press produced early revisions in the act that introduced deep ambiguities in its meaning, as well as a complicated jurisprudence that tried to clarify badly written law. Unfortunately, this laudable desire to protect the First Amendment had blind spots and unintended consequences.

The result was a patchwork system for policing state secrets — a runaway classification system, established with no congressional input, that gives meaning to sweeping, century-old prohibitions on disclosing information “related to the national defense.” The Espionage Act is 100. Perhaps it is time to revise it.