Which level of government provides for the common defense?
When we talk about “levels” of government in the United States, we are generally talking about the fact that we have a national government, state governments, and local governments. These are the three main levels of government in the US. While you could argue that all levels of government provide for some aspects of the "common defense,” we generally say that the national government is the level that does this. In fact, the preamble to the Constitution says that one reason for establishing the Constitution (for the national government) was to “provide for the common defence.”
You could argue that state and local governments also provide for the common defense. We have state National Guard units that help to do this. On the state and local level, we have police forces that can be said to be providing for the common defense against criminals.
However, when we talk about the common defense, we are usually talking about defense against other countries. The vast majority of this aspect of defense is provided by the national government because the national government is the level of government that controls our regular military forces.