When WikiLeaks founder Julian Assange released his latest document trove—more than 250,000 secret State Department cables—he intentionally harmed the U.S. government. The release of these documents damages our national interests and puts innocent lives at risk. He should be vigorously prosecuted for espionage.

The law Mr. Assange continues to violate is the Espionage Act of 1917. That law makes it a felony for an unauthorized person to possess or transmit "information relating to the national defense which information the possessor has reason to believe could be used to the injury of the United States or to the advantage of any foreign nation."

The Espionage Act also makes it a felony to fail to return such materials to the U.S. government. Importantly, the courts have held that "information relating to the national defense" applies to both classified and unclassified material. Each violation is punishable by up to 10 years in prison.

No doubt aware of this law, and despite firm warnings, Mr. Assange went ahead and released the cables on Nov. 28.

In a letter sent to Mr. Assange and his lawyer on Nov. 27, State Department Legal Adviser Harold Hongju Koh warned in strong terms that the documents had been obtained "in violation of U.S. law and without regard for the grave consequences of this action."

Mr. Koh's letter said that publication of the documents in Mr. Assange's possession would, at minimum:

• "Place at risk the lives of countless innocent individuals—from journalists to human rights activists and bloggers to soldiers to individuals providing information to further peace and security; 
• "Place at risk on-going military operations, including operations to stop terrorists, traffickers in human beings and illicit arms, violent criminal enterprises and other actors that threaten global security; and, 
• "Place at risk on-going cooperation between countries—partners, allies and common stakeholders—to confront common challenges from terrorism to pandemic diseases to nuclear proliferation that threaten global stability."

None of this stopped Mr. Assange. That he is breaking the law and must be stopped from doing more harm is clear. I also believe a prosecution would be successful.

In an October analysis of earlier WikiLeaks disclosures, the Congressional Research Service reported that "it seems that there is ample statutory authority for prosecuting individuals who elicit or disseminate the types of documents at issue, as long as the intent element can be satisfied and potential damage to national security can be demonstrated."

Both elements exist in this case. The "damage to national security" is beyond question. As for intent, Mr. Assange's own words paint a damning picture.

In June, the New Yorker reported that Mr. Assange has asserted that a "social movement" set on revealing secrets could "bring down many administrations that rely on concealing reality—including the U.S. administration." The same piece revealed Mr. Assange's stunning disregard for the grave harm his actions could bring to innocent people, which he dismisses as "collateral damage."

Mr. Assange claims to be a journalist and would no doubt rely on the First Amendment to defend his actions. But he is no journalist: He is an agitator intent on damaging our government, whose policies he happens to disagree with, regardless of who gets hurt.

As for the First Amendment, the Supreme Court has held that its protections of free speech and freedom of the press are not a green light to abandon the protection of our vital national interests. Just as the First Amendment is not a license to yell "Fire!" in a crowded theater, it is also not a license to jeopardize national security.

This latest WikiLeaks release demonstrates Mr. Assange's willingness to disseminate plans, comments, discussions and other communications that compromise our country. And let there be no doubt about the depth of the harm. Consider the sobering assessment, delivered in an email to employees of U.S. intelligence agencies late last month, by Director of National Intelligence James Clapper: "The actions taken by WikiLeaks are not only deplorable, irresponsible, and reprehensible—they could have major impacts on our national security. The disclosure of classified documents puts at risk our troops, law enforcement, diplomats, and especially the American people."