
Three key facts of the declaration
A study of the Declaration of Independence reveals three key facts.
1. The colonies declared themselves to be “free and independent states“

3. The declaration dissolved all political connection between the free and independent states and the British Crown.
The declaration
In Congress, July 4, 1776.
THE UNANIMOUS DECLARATION of the thirteen united STATES OF AMERICA
"We, therefore, the representatives of the united States of America, in General Congress, assembled, appealing to the Supreme Judge of the world for the rectitude of our intentions, do, in the name, and by the authority of the good people of these colonies, solemnly publish and declare, that these united colonies are, and of right ought to be free and independent states; that they are absolved from all allegiance to the British Crown, and that all political connection between them and the state of Great Britain, is and ought to be totally dissolved; and that as free and independent states, they have full power to levy war, conclude peace, contract alliances, establish commerce, and to do all other acts and things which independent states may of right do."According to the World Book Encyclopedia the "Articles of Confederation was the agreement under which the thirteen original colonies became the United States of America. They were the supreme law of the land from March, 1781 until the adoption of the Constitution in 1789.” However, this is only half the truth.
Perpetual Union is the key

Another notable insight is that the process to draft the Declaration of Independence began on June 11, 1776, while a parallel process to draft the Articles of Confederation and Perpetual Union was set in motion on the following day, June 12, 1776. The draft Declaration was ready for discussion in the Continental Congress by July 1. The decision to declare independence was approved on July 2 and the final wording of the text was accepted by 12 of the 13 states on July 4, 1776 - with New York giving its consent one week later. The Declaration was not signed by most of the states representatives until August 2, with the affixing of all signatures taking longer. By comparison, the draft of the Articles was completed in the autumn of 1777, when it was sent to the Congress for its approval. The final version of the Articles was approved by Congress on November 15, 1777 and sent out to the thirteen free and independent states for their ratification. That process was completed in early 1781, when the new nation came into being.
It is a historical incident that made it so that the Union gained its unanimous assent with Maryland becoming the thirteenth state to pass an act by its state legislature declaring its decision to ratify the Articles. The decision made the Union a reality. This is so even if a formal affixing of the signature of state delegates to the Articles took place only later, with the new Constitution entering into effect on the following day - when the Second Continental Congress reconvened as the United States in Congress Assembled. It was the cementing of the Union that made the USA a country and not the approval of its first constitution. The story of Maryland's ratification is explained in detail here.
SAMVA USA chart (Perpetual Union)

References
[1] Image of the handwritten original copy of the Declaration of Independence
http://upload.wikimedia.org/wikipedia/commons/0/07/Us_declaration_independence.jpg
[2] Image of the handwritten original copy of page one of the Articles of Confederation and Perpetual Union.
http://upload.wikimedia.org/wikipedia/commons/b/b4/Articles_page1.jpg