When did the united states became a state?
No context provided
When did the united states declare itself an independent state?
July 4, 1776
No context provided
When did the united states become a state under ratification of its first constitution?
March 1, 1781
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When did the united states become a state under an effective Constitution?
March 4, 1789
History of the United States
https://en.wikipedia.org/wiki/History%20of%20the%20United%20States
The Articles of Confederation and Perpetual Union was an agreement among the 13 original states of the United States of America that served as its first constitution. 
Articles of Confederation
On July 2, 1776, the Second Continental Congress, meeting in Philadelphia, declared the independence of the colonies by adopting the resolution from Richard Henry Lee, that stated:

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; that measures should be immediately taken for procuring the assistance of foreign powers, and a Confederation be formed to bind the colonies more closely together.
History of the United States
On July 4, 1776, they adopted the Declaration of Independence and this date is celebrated as the nation's birthday. 
History of the United States
The Second Continental Congress, meeting in Philadelphia, declared the independence of the colonies by adopting the resolution from Richard Henry Lee. On July 4, 1776, they adopted the Declaration of Independence. The Articles of Confederation and Perpetual Union was an agreement among the 13 original states of the United States of America that served as its first constitution. It was ratified on March 1, 1781. the United States Constitution took effect on March 4, 1789.