Final answer:
The most significant accomplishment of the Second Continental Congress was the Declaration of Independence, which formally declared the American colonies' independence from Great Britain and became the foundation for the United States.
Step-by-step explanation:
The most significant accomplishment of the Second Continental Congress was the Declaration of Independence. This document, which was adopted on July 4, 1776, formally declared the American colonies' independence from Great Britain. It outlined the principles of individual rights, self-government, and the pursuit of happiness, which became the foundation for the newly formed United States of America.