Why was the Declaration of Independence written?

The main reason why the Declaration of Independence was written was to explain to foreign countries why the colonies wanted to separate from Great Britain.  The colonies hoped that other countries would support them in their struggle once they learned why the colonies wanted independence.

The colonies were clearly going to have a hard time winning independence from Great Britain all by themselves.  Great Britain was the major military power in the world and the colonies were not even a country.  To win a war, they were going to need some help.  To get this help, they needed to convince other countries that their cause was just.  Therefore, they decided to write the Declaration.  The Declaration was meant to lay out their reasons for rebelling so that other strong countries would be inclined to help them.  We can say that they were successful in this attempt because France, in particular, gave the colonies very important assistance in the Revolutionary War.