Final answer:
The main purpose of the Declaration of Independence was to declare the American colonies' separation from Great Britain, justify the separation by outlining the democratic principles of equality and unalienable rights, and to seek international support for the American Revolution.
Step-by-step explanation:
The main purpose of the Declaration of Independence was to formally express the American colonies' intention to separate from Great Britain and to provide the rationale behind this decision. The document outlines a philosophical justification for independence, arguing that when a government fails to protect the unalienable rights of the people - life, liberty, and the pursuit of happiness - the people have the right to alter or abolish it and establish a new government. The author's main purpose was thus to announce and justify the colonies' break from British rule and to encourage support for the American Revolution.
The Declaration expresses democratic ideals by asserting that all men are created equal and have certain inalienable rights. This assertion was a groundbreaking concept at the time, setting a new standard for government that is based on the will and consent of the governed, rather than the rule of a monarch.
In summary, the author's main purpose was to declare independence from Britain, state the democratic principles that justify such an action, and to mobilize international support.