(CNN)He stood 5-foot-4 and weighed 130 pounds. An angular, baby-smooth face made him look even less intimidating. Henry Johnson was at first just another railroad porter who toted luggage and smiled for tips.
From Syria to Black Lives Matter: 3 ways WWI still shapes America
But on May 4, 1918, Johnson grabbed a two-inch bolo knife and a splintered rifle and did something so remarkable that he earned another name: "Black Death."
Johnson was a US sergeant standing sentry one night in a French forest when a German raiding party attacked. The swarming Germans shot Johnson in his lip, head and side. Yet Johnson kept shooting back. When his rifle jammed, he grabbed it by the barrel and clubbed more Germans. Then he used the bolo knife to stab and disembowel another enemy soldier. He kept throwing grenades until he fainted from blood loss.
When his comrades found Johnson the next morning, they discovered he had killed four Germans and wounded about 20 more. They could still see the bloody trails of wounded Germans who had crawled into the woods to escape Johnson's fury. Johnson had been wounded 21 times but somehow survived the hourlong battle.
"There wasn't anything so fine about it," Johnson would say later when praised for his gallantry. "Just fought for my life. A rabbit would have done that."
Johnson's story is featured in PBS' "The Great War," a stirring account of America's entry into World War I. The three-part "American Experience" film, which begins airing Monday, devotes six hours over three nights to explaining why the nation decided to enter "the war that would end all wars" 100 years ago this month.
Johnson's story captures what's distinctive about the film. He was a black soldier who faced something even more lethal than German bayonets when he returned home. He discovered an America that was also at war with itself. Some of the most ferocious battles during World War I took place not in Europe but on the streets of America -- and some are still being fought today.
What should the President do when a foreign dictator is accused of murdering women and children? Does the US welcome too many immigrants? Are corporations too powerful? Are women treated like second-class citizens? Those might seem like questions ripped from today's headlines, yet they literally provoked riots and lynch mobs during World War I, the film shows.
Few people today, however, know how relevant the war remains because it seems so distant, trapped forever in wobbly black-and-white silent film, historians say.
"The First World War is the most important event that most people don't know about," says Dan Carlin, a historian whose "Hardcore History" podcast examines World War I. "It's a Pandora's box. We're still ironing out what it unleashed."
Here are three battles from "The Great War" the United States is still waging:
They speak in funny accents and don't care about fitting in. So many are pouring across the border that they're threatening the American way of life. They're not real Americans.
That's what many Americans thought of German-Americans during World War I.
If you think political battles over immigration are tough today, they were vicious when America entered World War I, "The Great War" shows. A wave of hysteria aimed at German-Americans swept the nation as it struggled to assimilate what was then its largest ethnic group.
America didn't just declare war on Germany -- it waged war on German-American culture. Newspapers warned of "German troublemakers" and "German traps." People refused to drink German beer, and children were instructed to rip German songs out of music books. In one Ohio town, officials slaughtered all dogs belonging to German breeds.
A German-American coal miner accused of being a spy was even attacked by a mob, stripped of hi