The nation's defense depended on state militias under the Articles of Confederation. The federal government has the power to maintain armed forces to protect the country and its citizens.
According to the Declaration of Independence, the purpose of government is to promote "Life, Liberty, and the Pursuit of Happiness" for the people of the nation. To "promote the general Welfare," maintain order, protect individual liberties, regulate bankruptcies, and promote science and technology.
The Framers believed that preserving liberty should be major goal of the Constitution. The Constitution guarantees that no American's basic rights will be taken away now and for the generations not born yet. (posterity)