On Dec. 4, 1941, three days before the attack on Pearl Harbor, Frank Knox, the secretary of the Navy, assured those gathered at a private dinner party of administration officials that wherever the Japanese struck, “the Navy is ready … it is not going to be caught napping.”

Of course, he was wrong, with tragic consequences on Dec. 7, 1941, a day Americans commemorated last week, in the 75th anniversary of the bombing of Pearl Harbor. He was also humiliated, with members of Congress calling for his head.

Perhaps to head off a full-scale congressional investigation, on Dec. 8, Knox flew out to Hawaii “to see for myself just what happened” — determined, he wrote a friend, to clear up “a cloud of rumors” and then “to tell the whole truth and put the blame exactly where it belongs.”

But when he returned on Dec. 15, 75 years ago this week, he did neither.

At a news conference that day packed with reporters, he put the blame on Japanese Americans living in Hawaii. He provided no evidence for his claim, then or ever.

Nor could he have, as there was none, according to the most thorough government review ever of the internment of 120,000 Japanese Americans, according to the report of the Commission on Wartime Relocation and Internment of Civilians. What’s more, other intelligence officials in Washington and Hawaii, including the FBI and Army intelligence, disputed Knox’s claim in private.

They said nothing in public, however, and after Knox’s news conference, the press went wild with screaming headlines about a treacherous “fifth column” of Japanese in the United States, the very words used by Knox. “Japanese Spies Showed the Way For Raid on Vital Areas in Hawaii,” reported United Press International.

That made him the first official of the U.S. government to put his weight and office behind what had been a widespread and hysterical frenzy of unfounded claims against people of Japanese descent living in the U.S. He told the public what it wanted to hear.

“It was a more innocent time,” said University of Quebec-Montreal historian Greg Robinson, author of “By Order of the President: FDR and the Internment of Japanese Americans,” in an interview with The Washington Post.

“People assumed that if someone” high in government “makes an authoritative statement, they knew what they were talking about.”

“With his words,” writes University of San Francisco historian Christopher D. O’Sullivan in his upcoming biography of Knox, the wartime secretary of the Navy “played a major, ignominious role in one of the most egregious civil rights violations of the war: the internment of Japanese Americans.”

“The alarm Knox had rung gave immediate credence to the view that ethnic Japanese on the mainland were a palpable threat and danger,” concluded the 1982 report of the Wartime Relocation Commission, which recommended reparations for those interned, which were ultimately paid by an act of Congress.

“The damage was remarkable.,” the report also said.

Why remember it now? There are “disturbing” parallels with events today, said Robinson: the willingness of people to accept as fact stories with no evidence; the singling out of an “other” — now American Muslims; and fear.

Filmmaker Frank Chi captures young Muslim Americans reading letters written by Japanese Americans during their time held in internment camps. (Frank Chi/ Smithsonian Asian Pacific American Center)

More people, he suggested, know about the circumstances of internment from conservative provocateur Michelle Malkin than they do from actual history. “In truth,” she wrote in her book, “In Defense of Internment: The Case for ‘Racial Profiling’ in World War II and the War on Terror,” “the U.S. government’s national security concerns during World War II, particularly the threat of espionage in support of the Japanese emperor, were real and urgent.”

It would be inaccurate and unfair to suggest that Knox was the prime mover for the internment of 120,000 Americans of Japanese descent during World War II, 70 percent of whom were U.S. citizens. It was a collective decision backed by President Franklin D. Roosevelt’s executive order, with little opposition from around the country and ultimately upheld by the U.S. Supreme Court.

But he certainly helped fuel public support for it.

Talk, but no evidence, about Japanese American sabotage in the U.S. were already rife, as was anti-Japanese American prejudice, particularly on the West Coast and within the government itself.

Indeed, it was the military commander for the West Coast, Lt. Gen. John L. DeWitt, head of the Army’s western command, who wrote in a report these words:

In the war in which we are now engaged racial affinities are not severed by migration. The Japanese race is an enemy race and while many second and third generation Japanese born on United States soil, possessed of United States citizenship have become ‘Americanized,’ the racial strains are undiluted.

And some Japanese Americans had already been rounded up.

But while conspiracy theories were rampant, no ranking federal official had publicly given them credence until Knox, the secretary of the Navy.

The commission report expressed some puzzlement about why Knox did what he did.

But the chapters about Knox’s behavior from O’Sullivan’s forthcoming Knox biography, which he made available to The Post, and the documents he cites, provide some convincing answers.

He “had long possessed negative views towards Asians and, in particular, Japanese, common for his time,” writes O’Sullivan. Knox wrote in March 1933, according to O’Sullivan, that “the first military precaution to be taken in Hawaii is to intern every Japanese resident before the beginning of hostilities threatens.”

After becoming secretary of the Navy, he had “consulted the Army and the Justice Department about establishing a national infrastructure of concentration camps,” O’Sullivan writes, “presumably to detain Japanese nationals and Japanese Americans.”

And, after bragging of the Navy’s readiness, he had been caught off-guard by the attack on Pearl Harbor and humiliated by it.

He had also personally assured President Roosevelt. At an emergency Cabinet meeting on Dec. 5, 1941, he told him he knew the whereabouts of the Japanese fleet and it wasn’t anywhere near Oahu.

“Well, you know Mr. President, we know where the Japanese fleet is,” Knox told FDR, according to O’Sullivan.

“Well, you tell them about it Frank,” said Roosevelt, reports O’Sullivan.

And he did. “We know it’s in the ocean,” he said. “At sea.”

But the “sea” to which Knox referred was near Malaya. “He had no idea that a Japanese carrier task force had been set in motion in mid-November and was now bearing down on Pearl Harbor,” O’Sullivan writes. “Naval intelligence had been watching the wrong fleet ….”

It was a bad week in Washington for everyone. But it was a worse week for Frank Knox.

O’Sullivan quotes oral histories of those in attendance as Roosevelt’s Cabinet began getting reports of the attack. It was mortifying for Knox.

“Mr. President, several of us have just arrived by plane,” said Attorney General Francis Biddle. “We don’t know anything except a scare headline ‘Japs Attack Pearl Harbor.'” Roosevelt asked Knox to tell the story and twice snapped at Knox: “Find out, for God’s sake, why the ships were tied up in rows.”

“That’s the way we berth them,” Knox said.

A senator in the meeting, Tom Connally of Texas, “exploded at this,” writes O’Sullivan. Knox “looked like he was attending the funeral” of a family member, a witness reported.

After the attack on Pearl Harbor, Knox was a prime target of criticism. Roosevelt told confidant Bernard Baruch that “he was going to get rid of a big man in the Administration,” and that that man was Knox, O’Sullivan reports. Members of Congress were calling for his head.

On his hurried trip to Hawaii, O’Sullivan writes, Knox was shocked at what he saw and learned, that “the attack had been a complete surprise to both the Army and Navy. Japanese success was due largely to an inexplicable lack of readiness.” As a longtime advocate of naval preparedness — his claim to fame — it must have been doubly devastating to Knox.

The term “fifth column” was a familiar one in the U.S. Though it originated during the Spanish Civil War in 1936, the public knew it perhaps more intimately from the war in Europe, where a “fifth column” of collaborators in Norway, most notoriously Vidkun Quisling, had aided the Nazi invasion of that country.  So Knox knew full well how his words at the news conference upon his return from Hawaii would be understood and resonate.

“The most effective fifth-column work in this war was done in Hawaii, with the exception of Norway,” he told reporters. “The net of it was that pretty complete information had been given to the Japanese. They had the most perfect information about the defense of the island, the disposition of the fleet and everything else.”

He reported the same to FDR.

His motives: “We can only speculate,” writes O’Sullivan. “… For Knox, it may have been more convenient to lay the blame on alleged fifth columnists rather than accept that the Navy, and by extension, Knox himself, had been caught totally unprepared.”

“Knox was trying to get the Navy off the hook,” Robinson said. “In his case it was to shift blame from himself and the Navy.”

For all their supposed “fifth column” activities, Japanese Americans in Hawaii were not interned en masse. They represented 40 percent of the population, including most of the island’s carpenters and transportation workers. They were needed to rebuild Pearl Harbor.

Most of those interned lived on the West Coast.

Congress awarded modest reparations, up to $20,000 to survivors of the internment camps — most of whom lost homes or farms and businesses, many of whom were dead — and in 1988, President Ronald Reagan signed an apology, on behalf of the American people.

Knox would not live to answer for fueling the hysteria of the period or perhaps to explain how he arrived at the notion of a “fifth column” of Japanese Americans in Hawaii.

Knox, a onetime Teddy Roosevelt “Rough Rider,” publisher of the Chicago Daily News and  Republican Alf Landon’s vice-presidential running mate in 1936, died in 1944 of a heart attack at the age of 70.

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