The mention of confederate states is spot on!
First, this was written in 1787, way before the Confederacy of the Civil War.
Second, when Madison writes of "confederate states," he's referring to the Articles of Confederation that was the current organizing document for the states. He's saying that other states in the confederation would join the state being assaulted by the federal government to block the overreach of federal power into the states.
-PJ
As I said: Spot on! You seem to presume that I am unknowing... 🤔