The Constitution made a number of significant changes in the structure of the Federal government, all aimed at making the United States a single political entity rather than a "confederation" of separate states. The Constitution made it possible, for example, for the Federal government to collect taxes directly; under the Articles, taxes could only be collected by the states. The Federal government became directly responsible for the military, whereas under the Articles the Federal government had to request troops from each state. The Constitution also created the Federal judiciary, including the Supreme Court, which made the Federal government the final arbiter in disputes between states. Perhaps most important, the Constitution created the presidency, a Federal executive who was given the power to make decisions on behalf of all the states -- a single person who could be identified as the "leader" of the nation.
There were many problems with the Articles of Confederation. The national government did not have the power to levy taxes on the people. Instead, it had to ask the states for money and hope that they complied. The government did not have an executive branch led by a chief executive who could actually lead the country. It did not have the power to maintain a standing military. The states had a great deal of power over the national government and could veto its actions. The states were practically independent and, therefore, were able to do things like engaging in trade wars with one another. All of these weaknesses were fixed by the Constitution.
The Constitution gave the federal government the power to “lay and collect taxes.” It created a separate executive branch of the government that would be headed by a single president with relatively important powers. It had the power to “raise and support” an army and to “provide and maintain” a navy. The states no longer had the power to veto actions by the federal government. The Congress had the power to regulate trade between the states, thus making the states less like independent countries and less able to engage in trade wars with one another. In these ways, the Constitution strengthened the federal government in areas where the Articles of Confederation had left it weak.