In the future, 4 friends go on a field trip into the 1940s. When they return, the future has changed greatly, which now has the United States under dictatorship. Now the boys must find out what happened and who changed the world.

What Richiev said. The joy ride into the past has to the cause of the present calamity - the inciting incident for the plot -- otherwise what purpose does it serve?

The story could be about ironical intention: the protagonist and his friends just want to have a good time in the past, but --"Doh!" --their joy ride has unintended consequences. Their intervention in the past creates the mess; now they have to figure out a way to clean it up, save the world from their short-sighted folly.

What Richiev said. The joy ride into the past has to the cause of the present calamity - the inciting incident for the plot -- otherwise what purpose does it serve?

The story could be about ironical intention: the protagonist and his friends just want to have a good time in the past, but --"Doh!" --their joy ride has unintended consequences. Their intervention in the past creates the mess; now they have to figure out a way to clean it up, save the world from their short-sighted folly.

Traveling to the past and changing the future for worse is standard fare in science fiction. I would try coming up with a hook that grabs the reader.

An Example:
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"After accidentally sleeping with his great grandmother, a time traveler must destroy the temporal circle he created before the fabric of time is rent asunder and the universe ceases to exist."
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Of course the end of this story would be, he realizes if the universe is to be saved, he must kill himself. :) A happy story!

But that would just be an example

Good luck with your story, find a hook!