TRENTON — The entrenched armies of Europe were mowing each other down along a 400-mile front in Belgium in early 1915.

But on American shores, 100 years ago today on a cold January night, a mysterious blaze reduced an important Trenton cable factory to ruins.

German agents were suspected in the fire at the Roebling Works, though no one was arrested and German involvement was never proven.

The mystery blaze on the night of Jan. 18, 1915 marked the beginning of a series of 200 fires and explosions that terrified Americans and were centered in industrially-powerful New Jersey, bringing World War I home to America.

The state was the front line in an undeclared war that would burn for more than two years. It included explosions in Pompton Lakes, Carneys Point and Jersey City, a munitions plant in Lyndhurst and, most famously, the massive detonation of the Black Tom munitions depot on the New Jersey side of New York Harbor.

“New Jersey was the center for so much spy activity, from the cells in Hoboken to Black Tom to the boarding houses where schemes were plotted,” said Howard Blum, a writer and historian who published the bestselling “Dark Invasion” about German wartime espionage last year.

“Everything came through New Jersey ports,” added Jules Witcover, a journalist who wrote “Sabotage at Black Tom” about the terror campaign. “Everything.”

The Roebling company in Trenton had just begun to make vast profits off the European war by making strong metal cable which was used in heavy shipping. Its cable was used in the construction of the Brooklyn Bridge.

“The fire alarm was systematically cut so that fifteen minutes were lost waiting for fire engines. It started in a room where no work was being done, but there were quantities of cotton, jute, etc., stored there, all inflammable, and lastly a man was seen running away from the room,” according to “The Roebling Legacy," published in 2011 by historian Clifford Zink. "The mystery remains."

A century later, historians still debate whether it was set by German conspirators. David Petriello, a Caldwell College history professor who published a new book, "Military History of New Jersey," last August, said it’s only common sense that the Kaiser’s agents were responsible.

“The alarm wires had been cut so we can safely assume it was sabotage. This leaves two culprits, the Germans or Anarchists, who rarely targeted workers,” said Petriello.

Henry Landau, a captain in the British Secret Service, published a book called "The Enemy Within" in 1937. In it he claims the German spy network began its two-year reign of terror with the attack on Roebling.

“Soon the papers were filled with accounts of mysterious fires and explosions in ships and factories,” wrote Landau. “A few of the disasters can be written down to accidents or carelessness due to the sudden increase in the manufacture of munitions, but German sabotage agents were undoubtedly responsible for the bulk of them.”

But there were other suspicious attacks, right as Europe began its cataclysm. German agents allegedly blew up a DuPont powder plant in Pompton Lakes on Aug. 30, 1914, and bombed the Detwiller and Street fireworks plant in Jersey City in October 1914, killing four workers.

"All of a sudden, the war was brought home," said Randall Gabrielan, author and the Monmouth County historian. "Germany regarded us as an enemy - and rightfully so."

By 1915, a vast conspiracy involving the highest echelons of the German diplomatic corps had taken root, and had set their sights on New Jersey, according to some historians. It was a shadow war that was to last until the U.S. entered the war.

The biggest targets were in New Jersey. The attack on the Black Tom pier in Jersey City on July 30, 1916, was a wake-up call to a neutral and somewhat naïve nation, according to the historians. German agents lit fires on the pier in New York Harbor, which was the biggest munitions depot on the East Coast, with a stockpile of between 2 and 4 million pounds of explosives.

That entire load ignited at 2:08 a.m., with the resulting blast killing at least five people in the region and causing damages amounting to $400 million in today’s dollars.

“The Black Tom explosions physically shook the New York area and psychologically rocked a nation believing itself to be far removed from the terrors of the Great War,” writes Witcover in "Sabotage at Black Tom."

Another attack was made on the Canadian Car and Foundry plant in Kingsland, now known as Lyndhurst, on Jan. 11, 1917. The U.S. was still neutral, but the 40-acre compound in the Meadowlands was producing roughly three million artillery shells per month, mostly bound for the Russian Army on the Eastern Front, according to Petriello.

A German agent got hired there and set a fire, which was accelerated by as many as half a million bursting artillery shells, according to accounts. A woman working the switchboard stayed at her post, spreading the alarm, which was partially responsible for saving the lives of all 1,700 employees – many of whom fled across the frozen Hackensack River.

German authorities have only admitted their involvement in the Black Tom blast— in 1953 — when they paid $50 million in reparations. A spokesman for the German consulate this week said the government in Berlin was opting not to comment on the long past.

"We don't comment on historical events that happened 100 years ago," said Jens Alberts, the spokesman for the German consulate in New York.

Historians said the German network was broken up in the fallout of the Black Tom and Kingsland explosions, with a handful of the leaders deported or arrested.

Partly due to the outrage over the attacks, the U.S. joined Great Britain and France on the Allies' side of the war. The attacks arguably stopped when war was declared - since espionage and terror attacks became offenses punishable by the death penalty.

"The importance of these attacks is to show that regardless of the war or century, New Jersey will always be on the front-lines of conflict due to its geography, industry, and demographics," Petriello said. "From the various French and Indian Wars and the Revolution straight through to the attacks of 9/11, NJ is not immune to the violence and intrigue of hostilities."

Map created by NJ Advance Media reporter Stephen Stirling

Seth Augenstein can be reached at saugenstein@njadvancemedia.com. Follow him on Twitter @SethAugenstein. Find NJ.com on Facebook