In the years leading up to World War I, audiences flocked to silent movie theaters and danced to ragtime. But an affluent young man was just as likely to spend his summer vacation preparing for war as learning the Turkey Trot or the tango. Beginning in 1913, thousands of American men went to summer boot camps—volunteers in a growing movement to prepare the United States for what seemed like an inevitable war.

They had an influential ally: former president Theodore Roosevelt. He had plenty of experience with volunteer troops; after all, he’d organized and led the Rough Riders, a volunteer cavalry, during the Spanish-American War two decades earlier. Roosevelt thought that current President Woodrow Wilson was too soft on Europe and pushed him to prepare for war.

Wilson, who had defeated Roosevelt in 1912, was suspicious of combat. He preferred neutrality, pushing an image of the United States as an impartial peacemaker that could broker a ceasefire against Europe’s feuding factions. However, a growing faction of Americans disagreed—and started to push Wilson to do more. Their movement was called “preparedness,” and the focus was on getting America’s young men in fighting form, just in case.

The concept was simple: Men would give their summer vacations to their country and emerge prepared for eventual war. Eventually, 40,000 young men attended Plattsburg Camps—named after the first training camp in Plattsburgh, New York—nationwide with the aim of becoming officers if war was declared,

Starting in 1913, affluent young men ditched their leisurely summer plans and headed to boot camp instead. Over the 90 days of camp, attendees rose to an early-morning bugle, then spent the day doing drills, calisthenics and other activities. Their training culminated in “the hike,” a grueling multi-day ordeal that pitted recruits against one another in simulated battle. But the demanding, physically taxing schedule didn’t seem to dampen recruits’ enthusiasm. According to historian John Garry Clifford, they were so eager to learn that officers had to remind them to stop drills and take time off.

That enthusiasm wasn’t limited to actual recruits. Preparedness offered a heady combination of patriotism and pageantry—a sense that even though war seemed inevitable, it could be mastered. By 1916, the idea was so popular that 145,000 people gathered in its favor in a New York parade that took hours. Songs like “On to Plattsburg, March!” and “Prepare the Eagle to Protect the Dove” declared their willingness to fight.

Supporters of the Plattsburg idea, as it was called, thought the country was better safe than sorry. They felt that the United States lacked “the sense of personal obligation to do something for the country,” said Grenville Clark, a lawyer who helped organize the early camps and who later won an Army Distinguished Service Medal during World War I. To Clark and others, military training seemed like something the country could do during a time of foreign policy paralysis.

But not everyone was excited about preparedness. A similar parade in San Francisco was targeted by a radical group that killed 10 people and wounded 40 more with a suitcase bomb. And though their tactics were less extreme, pacifists like Jane Addams warned that preparing for war would simply set the stage for a catastrophic conflict.

“We believe in real defense against real dangers, but not in a preposterous ‘preparedness’ against hypothetical dangers,” wrote Addams and other anti-war activists in a 1915 letter to Wilson. A military buildup, they wrote, would foment suspicion and make it harder to broker international peace. Conscientious objectors and peace advocates argued that by pursuing military preparedness, the United States was simply emulating Germany, which had compulsory military service.

But supporters of preparedness had little patience for pacifism. Instead, they urged Wilson to adopt universal military training, or UMT. Clark and others appealed to Congress, but Wilson resisted. However, he did increase the size of the military.

Ironically, the preparedness movement did little to ready the United States for war. After Wilson reluctantly declared war on Germany on April 2, 1917, the number of volunteers wasn’t enough to satisfy the gigantic number of troops called for by General John J. Pershing. Six weeks after declaring war, Wilson’s Congress passed the Selective Service Act, and 2.8 million men were eventually drafted into World War I. Meanwhile, Wilson turned down Roosevelt’s bid to join the war with a volunteer corps of his own.

Preparedness may not have exactly worked, but the camps that supplied so many of the war’s troops—90,000 Reserve officers in all—survived. The Plattsburg Camps evolved into the Citizens’ Military Training Camps, which took place every summer from 1921 through 1940. Four hundred thousand men, including future president Ronald Reagan, attended the camps. But the Selective Service, which has registered 89 percent of all American men between the ages of 18 and 25, remains World War I’s most potent modern military legacy.