What if the Japanese battleships got past the big guns that were the key coastal defenses around San Francisco and the Golden Gate? What then? The U.S. Army had an answer. On the night of Dec. 7, the Army assigned every available soldier at the Presidio of San Francisco to get to work digging slit trenches and field fortifications to stop a Japanese invasion.

Trenches were dug on the bluffs above the Golden Gate. Machine guns were sited to cover Baker Beach on the western edge of the city. If the Japanese came, we were ready.

Nearly 65 years went by, and the world changed. The Army is gone from the Golden Gate. The Presidio is part of a national park now. The other day, National Park Service crews clearing weeds and making surveys for a hiking trail above Baker Beach found some of the old wartime trenches and machine-gun nests, still there, still ready for the invasion that never came.

The rangers were amazed. "It's hard to describe the experience," said Park Service historian Stephen Haller. "It's peeling back history."

The Park Service doesn't want to reveal the exact location of these trenches until archaeologists can look at them and prepare them for public viewing. There are perhaps a dozen trenches, on the bluffs north of Baker Beach, behind "keep out" signs.

The fear of those dark winter days in 1941 and 1942 seems nearly absurd now. The Japanese had no plans to invade and no fleet ready to mount an invasion _ a good thing, since the West Coast was defenseless. The Navy was out in the Pacific, and the Army was undermanned and unprepared. At one point in early 1942, Boy Scouts were sent to guard the Bay Bridge.

Retired ranger John Martini remembers taking an oral history from an old soldier named Dudley Riggs who had been stationed at the Presidio. "They gave me a World War I Army helmet, some ammunition dated 1920, a 1903 Springfield rifle and told me to shoot anyone coming up the hill," Riggs said.

On the afternoon of Dec. 7, the Army's Western Defense Command received a report of a Japanese fleet 30 miles off the Presidio. On Dec. 8, aircraft carriers were reported off the coast and a submarine off the Golden Gate, and at 6 that night, something suspicious was spotted on radar 100 miles west of San Francisco.

Sirens wailed, that eerie rising and falling sound that still signifies an air raid.

Cars and electric commuter trains were stopped on the Bay Bridge. Traffic stopped in the city, people piled out of buses and streetcars and took shelter. It was the war's first blackout on American soil, and it was a fiasco.

Many neon advertising signs stayed lit. Downtown San Francisco sparkled, one resident said, "like New Orleans at Mardi Gras time." The roadway lights and the rotating red beacon lights on the 4-year-old Golden Gate Bridge blazed away. The bridge, it was learned later, was defended by only three .30-caliber machine guns.

The next day, Lt. Gen. John de Witt, head of the Western Defense Command, came to City Hall to chew out the city fathers. He was in uniform, three silver stars glittering on each shoulder and blood in his eye. He was furious. He was convinced, he said, that Japanese bombers had flown over San Francisco _ and the city had not blacked out.

"No bombs fell, did they?" Mayor Angelo Rossi asked gently.

De Witt told the newspapers it might have been better if the city had been bombed. "I never saw such apathy," he snapped. "It was criminal. ... It was shameful."

There were no planes, but, according to Brian Chin's book "Artillery at the Golden Gate," there really were Japanese submarines off the coast.

They torpedoed a few ships off California and later shelled an oil refinery near Santa Barbara. On Dec. 17, Chin wrote, the submarine I-15 surfaced near the Farallon Islands. Its crew could see the glow of the city lights in the distance.

"If we weren't at war," said Capt. Hiroshi Imazato, "this would be an excellent chance to pass in through the Golden Gate and visit that famous city of San Francisco."

The Japanese officers all laughed.