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Music It's 2 p.m. on July 22nd, 1916 in San Francisco, California.

Marching along the street, 40-year-old Dr. Joseph Moss waves to the cheering crowds that packed the sidewalks. It's preparedness day, and a parade has been organized by San Francisco's Chamber of Commerce.

They want to show their support for an increase in defense spending by the government amid the ongoing war in Europe.

To his left and right, Dr. Moss is flanked by fellow veterans who fought in the Spanish-American War two decades ago.

Now, though, he's a prominent surgeon in San Francisco.

But Dr. Moss hasn't forgotten what he learned in the U.S. Army.

He marches in perfect step with those around him.

As a former soldier, Dr. Moss believes the United States has a responsibility to the rest of the world

and should join the fighting on World War I. 50,000 other marchers today agree with him. But many in America would

prefer to stay out of the conflict. Some of these isolationists have even made violent threats

against today's parade. But as he continues along Marcus Street, Dr. Moss doesn't think much

of these threats.

But then, out of the corner of his eye, he spots something falling from a rooftop up ahead.

It looks like a package or a small bag.

Dr. Moss can't make it out, but it's trailing smoke.

And as it lands among the crowd below, an ear-piercing explosion knocks Dr. Moss to the ground.

Through dust and smoke, Dr. Moss sees the vague shapes of men, women, and children, staggering and shock all around him.

Dr. Moss's medical and military training kicks in.

...