In September 1933, a small group of Nazis met in Los Angeles to discuss a secret plan: the takeover of America’s West Coast.

It was orchestrated by Dietrich Gefken, who had been an organizer for Adolf Hitler’s paramilitary Brownshirts back in Germany and a veteran of the 1923 putsch against the Bavarian government that had landed the politician in jail. Gefken had killed Jews in cold blood and “had not hesitated to throw acid in the faces of his enemies,” according to the book “Hitler in Los Angeles: How Jews Foiled Nazi Plots Against Hollywood and America,” by Steven J. Ross (Bloomsbury), out Tuesday.

Gefken spent the early 1930s traveling the US, working as a cook and spreading Nazi propaganda, before settling down in LA. Now, he was a leader in the secret storm-trooper unit of the local branch of Friends of the New Germany (FNG), a Nazi front organization laying the groundwork for an American invasion.

Having volunteered for the National Guard in San Francisco, Gefken had mapped out its Armory, which held enough weapons for an entire regiment, and connected with members of the Navy who would sell him other stolen weaponry.

Once that was secured, he planned to spur FNG storm troopers who had been “secretly training in street fighting and the use of bombs” to “launch spontaneous uprisings in San Francisco and San Diego.”

He strategized that these events would ignite similar rebellions along the West Coast, during which Gefken and his troops would use machine guns to “corral [American] Army officers and lock them up.”

“Nazis would confer with captured American troops,” Ross writes. “Those willing to pledge their loyalty to Hitler would be taken into the storm troopers. Those who refused would be killed on the spot.”

The plot was not idle fancy. Nazis had established strongholds and support in LA, including within the LAPD, whose chief, James Davis, once defended Hitler to a Jewish lawyer.

Fortunately, one man present at the takeover planning wasn’t a Nazi at all, but a spy who had infiltrated their ranks.

German-born US Army Capt. John H. Schmidt was a mole for Leon Lewis, a Jewish attorney who had established his own spy network, acting without government support. By the beginning of World War II, Lewis would become an unofficial arm of American intelligence — and their most valuable source of information for Nazis in this country.

The son of German Jewish immigrants, Lewis was born in Hurley, Wis., in 1888. He earned a law degree from the University of Chicago and was the first national executive secretary for the Anti-Defamation League. After moving to LA, he diligently tracked anti-Semitism in the area.

In 1933, the year Hitler became Germany’s chancellor, around 100 Nazis held their first meeting in LA to talk about unifying the area’s 150,000 Germans to promote Hitlerism in the US.

Lewis knew that decisive action needed to be taken.

Having done “secret intelligence work for the military during World War I,” he established a private espionage organization, finding spies who could infiltrate the Nazis and expose their devious plans. Knowing that Semitic-looking men or women would be recognized as such, he sought non-Jews, preferably those of German heritage. Schmidt was his first undercover operative.

The blond-haired, blue-eyed Schmidt began spending time at the Aryan Bookstore in LA’s San Pedro neighborhood. There, he found the prime topics of conversation were how Franklin Roosevelt was a tool of the Jews and how he should be replaced by a president with Nazi sympathies.

Had Lewis not taken it upon himself to stop the Nazis, LA in the 1930s could have been a much more frightening place

Schmidt soon became part of the FNG’s inner circle. All the while, he was funneling information to Lewis.

Based on the collected intel, agents for the Navy arrested “two Marine corporals who were selling government rifles and 12,000 rounds of ammunition to local [Nazi sympathizers],” and also “dismantled storm-trooper units.” Gefken’s plot was foiled.

But Lewis, “preferring to remain under the radar . . . never took credit for the subsequent arrests.”

At a time when America did not yet have espionage capabilities, Lewis spent much of the next 14 years as the Unites States’ most powerful intelligence weapon against the Nazis in America, Ross writes.

In the run-up to World War II, his agents heard talk about Nazi attempts to sabotage US aircraft factories — including claims that workers inside the local Douglas Aircraft factory had “taken bolts out of planes.” The Nazis, it turned out, had encouraged followers to apply for jobs at these factories and had many people on the inside.

A spy working for Lewis heard of the plan and sent it to naval intelligence before anything could happen. Security was beefed up, and the plan was thwarted.

Lewis continued in his secret crusading role until 1947. In retaliation, numerous Nazi and fascist groups targeted him in several murder plots along with Hollywood luminaries including MGM studio head Louis B. Mayer, Jack Benny, James Cagney, Eddie Cantor, Charlie Chaplin and Al Jolson. Some of these plots were foiled by Lewis, others fizzled out. Nazis never achieved any of their goals for the US on the scale that they had hoped for, but they helped build anti-Semitic sentiment in LA.

Lewis remained involved in Jewish groups and practiced law until his death from a heart attack in 1954. He was 65 years old.

Had he not taken it upon himself to stop the Nazis, LA in the 1930s could have been a much more frightening place.

“From August 1933 until the end of World War II, with few resources at their disposal, [Lewis and his] courageous undercover operatives continually defeated a variety of enemies — Nazis, fascists and fifth-columnists — bent on violence and murder,” Ross writes.

“Without ever firing a weapon, they managed to keep Los Angeles and its citizens safe.”