Black Tom Island explosion

"How did World War I begin?" As Michael Neiberg explained, there are two answers. There's the ten minute version, which ends up being so simplistic it borders on misrepresentation, snd then there's the semester-long version.
The question, "Why did the United States join it?" is similar. There are the short answers, which are wrong, and there are the long answers which don't fit nicely into a multiple choice problem.
While the sinking of the Lusitania and the Zimmerman telegram were certainly significant, they are only two among many factors. One that's arguably just as influential, though far less remembered, was the Black Tom island explosion.

On July 30th, 1916, German saboteurs set fire to the 2 million pounds of small arms and artillery ammunition that was stored in the freight cars there. Explosions spread the fire to a barge that was tied up there, prompting the detonation of another 50 tons of TNT.

That explosion was so huge it measured at least 5.0 on the Richter scale, and embedded so much shrapnel into the Statue of Liberty's upraised arm that it had to be closd to the public. (It was not opened again untill renovations were completed in 1986.
It also had far-reaching effects on the role of the federal government. The United States did not have a national intelligence service, which made the investigaton difficult, and service the United States only rudimentary communications security. In addition, there were no federal laws forbidding espionage or sabotage except during wartime. [Source]

With no evidence of sabotage, the only conclusion authorities could draw in the immediate aftermath was that it was an accident. However, the public noted the disturbing recent string of explosions involving munitions, and grew increasingly hostile to Germany. Further investigations revealed German agents to be the culprit, but a settlement could not be reached until 1953.

It also had an effect on risk assessments prior to World War II. Fearing fifth column attacks from Japanese-Americans, Franklin Roosevelt (who was assistant secretary for the Navy during the Wilson administration) authorized their internment after the attack on Pearl Harbor.

Examples like this show that history is often like a tree -- seemingly insignificant events, like roots, combine to have great influence. And the effects of an event, like branches, can spread out over time -- perhaps diminishing in strength, but never quite completely ending.