At 3:42 a.m. on December 7, 1941, Quartermaster R.C. Uttrick peered through his binoculars from the deck of the minesweeper U.S.S. Condor. By the pale light of a waning moon, the American sailor spied something unusual piercing the glassy skin of the Pacific Ocean less than two miles south of the entrance to Pearl Harbor. “That’s a periscope, sir,” Uttrick reported to the officer of the deck, “and there aren’t supposed to be any subs in this area.”

Hours before swarms of enemy aircraft descended out of the blue in a sneak attack upon the United States naval base on Oahu, five Japanese midget submarines were already lurking beneath the ocean surface to join in the assault on Pearl Harbor. The Japanese navy considered the 78-foot-long miniature submarines to be their secret weapon. Carrying two men and two torpedoes that had double the explosive charge of those borne by Japanese bombers, the battery-powered midget submarines could glide at 19 knots and operate in the waters of Pearl Harbor that were too shallow for conventional submarines.

The Japanese originally planned “Operation Hawaii” to be solely an air strike but modified it to combat test the newly developed midget submarines, which were to surface after the start of the aerial attack and fire torpedoes into the American fleet. Flight commander Mitsuo Fuchida, who led the air assault, believed the mini-submarines to be an unnecessary risk that could only endanger the secrecy of the entire operation if they were spotted by the Americans before the bombers arrived.

“There’s a tendency among military staff to make their plans more complicated than they have to be, and the Japanese navy, in particular, tended to over-plan its operations” Robert Citino, senior historian at The National WWII Museum, tells HISTORY. “The pilots who were actually going to carry out the raid were deathly afraid that if one of the midget subs were sunk or captured that they would give the surprise away. Pearl Harbor could only succeed if it was a complete bolt out of the blue.”

Shortly after midnight on the “date which will live in infamy,” the first of the five midget submarines was released seven miles from the entrance to Pearl Harbor after riding piggyback on a conventional submarine. The men inside the submersibles knew they were likely on a suicide mission, a point driven home by the swords and pistols they were issued to ensure they wouldn’t be taken alive.

Not long after the launch of the fifth midget submarine, Fuchida’s worst fears were fulfilled when the Condor’s quartermaster reported the periscope sighting to the destroyer U.S.S. Ward. Shortly before dawn, the destroyer received word that another American ship had spotted a submarine conning tower near the mouth of Pearl Harbor. As the sky brightened, the Ward approached within 50 yards of the suspicious craft and opened fire at 6:45 a.m., sending the submarine to a watery demise.

It was the opening shot of the Pacific War.

“We have dropped depth charges upon sub operating in defensive sea area,” the Ward reported at 6:51 A.M. Two minutes later the destroyer provided additional details: “We have attacked, fired upon and dropped depth charges upon submarine operating in defensive sea area.”

The reports slowly percolated up to higher authorities. Admiral Husband E. Kimmel, commander of the U.S. Pacific Fleet, was still getting dressed in his quarters awaiting confirmation of the report when the buzz of Japanese dive-bombers suddenly shattered the Sunday morning solitude around 7:55 a.m. The Army command and airfields on Oahu never received word about the enemy submarine in the ensuing hour before the start of the attack that would claim the lives of more than 2,400 Americans.

“The question is why didn’t the information make it up the chain of command,” Citino says. “The message is crawling up two chains of command at a time when not everyone is in the office. In addition, there have been false positives of submarines around Pearl Harbor in the previous few weeks so not everyone was sure what had actually happened.” Even had the information been relayed quickly, Citino notes that “it takes a while to put a fleet at rest into combat readiness.”

While the storm of bombs rained down from the skies, American sailors also reported torpedoes being fired from the water. After darkness descended upon Hawaii after a dark day, one of the mini-submarines sent a signal in morse code to its mother craft claiming it had successfully attacked an American ship. However, whether the midget submarines actually struck any part of the American fleet in Pearl Harbor is still a subject of debate today.

While four of the midget submarines ended up resting on the seabed, either struck by American ships or mechanical failure, the fifth was plagued by a defective gyroscope from the very start. It floundered three times on a coral reef as it attempted to penetrate Pearl Harbor. A misfired shot from the American destroyer U.S.S. Helm blasted it free but knocked the crew unconscious. When Ensign Kazuo Sakamaki awoke from a stupor caused by the concussion and noxious fumes inside his craft around midnight, his vessel had drifted to the other side of Oahu.

Sakamaki lit a fuse to scuttle his faulty ship and dived overboard, but even that maneuver failed. The fuse didn’t ignite, his crewmate drowned in the surf and Sakamaki washed up unconscious on a beach, only to awake to the stare of American Sergeant David Akui who took him into custody as Prisoner of War, No. 1.

To the dismay of the pilots who attacked Pearl Harbor, the Japanese military falsely credited one of the midget submarines with landing the fatal blow on the U.S.S. Arizona. The nine Japanese submariners who perished became heroes in Japan and received posthumous double promotions, while Sakamaki garnered only the scorn of many of his countrymen for failing to take the honorable route of suicide.

The salvaged Japanese submarine toured the United States to promote the sale of war bonds and is now on display at the National Museum of the Pacific War in Fredericksburg, Texas, a relic of the little-remembered aspect of the attack on Pearl Harbor.

“You have 300 aircraft in the sky and five midget subs,” Citino says. “Even if each one had a direct hit, there was so much more ordinance flying through the air than gliding under the seas. In the shadow of that, the submarines become a footnote.”