Why did the authors of the Constitution believe they could improve on the Articles of Confederation? To what extent did they succeed?
The opening lines of the Constitution contain the phrase, "to create a more perfect union." The framers of the Constitution were acting to preserve the nation because the Articles of Confederation were unworkable. The Articles had been written at the time of the war as an attempt to maintain a loose organization of states who were jealous of their own sovereignty. Desirable as this seemed, it was not workable. With no power to tax, the central government had no authority to pay its bills, including the military. The Newburgh Conspiracy, in which Washington's officers threatened to march on the Continental Congress if it were not granted taxing authority, is a prime example. A super majority of states (voting as states) were required to pass legislation and unanimity to amend. The Constitutional Convention was initially called for the purpose of amending the Articles. It is no small wonder that the delegates decided almost immediately that they could not be amended into a functional government. Benjamin Franklin expressed it best when he told the delegates, "We must all hang together or we will all most certainly hang separately."
They believed they could improve on it because the Articles set up a system where the central government was much too weak. The states had all the power in that system. For example, the national government couldn't even levy taxes. Instead, it had to ask the states for money, which they could refuse to give. It couldn't keep the states from erecting trade barriers against one another. It had very little power and this was (in the eyes of the Federalists who wrote the Constitution) a major problem for the country.