Answer:
The United States of America was first established by the Constitution, written at the constitutional convention. However, due to conflicts while writing the Constitution, the convention lasted for 5 months. The only reason the Constitution of ratified was a series of compromises.
The Great Compromise
While writing the Constitution, the founding fathers debated how the states would be represented in Congress. The New Jersey plan said that all states should have equal representation and was favored by small states. James Madison wanted to set up representation proportionate to population, which favored large states.
It wasn't until Roger Sherman proposed the Great Compromise (aka the Connecticut plan) that the two sides could reach an agreement. The Great Compromised proposed a two-house congress consisting of the Senate and House of Representatives, one of which would have equal representation and the other having population-based representation. This compromise was accepted and written into the Constitution.
Bill of Rights
During the creation of the Constitution, there were two factions, the Federalists and the Antifederalists. The Federalists argued for the ratification of the Constitution while the Antifederalists wanted to amend the existing Articles of Confederation.
One of the main concerns of the Antifederalists was that the Constitution would limit the rights of the people. So, the two groups were forced to reach a compromise. The Federalists promised that if the Constitution was ratified, then they would add a Bill of Rights to protect the people. This compromise is why states such as New York agreed to ratify the Constitution.
Without these compromises and many others, it's possible the USA would never have become the unified nation it is today.