Federalist No. 18
In Federalist No. 18, Hamilton and Madison team up to address the ongoing issue of the insufficiency of the present confederation to preserve the union.
One of the key takeaways from this essay is summed up in these words a weak government, when not at war, is ever agitated by internal dissentions, so those never fail to bring on fresh calamities from abroad. As proof of the pudding they offer, they recount a history of Greece and how the relatively weak union they formed eventually broke down as rival city states went to war. Had Greece ... been united by a stricter confederation, and perservered in her union, she would have never have worn the chains of Macedon, and might have proved a barrier to the vast projects of Rome.
The majority of the essay is spent recounting the histories of the Grecian republics, and the battle between Athens and Sparta and the Achaean league in particular, because it teaches more than one lesson. It emphatically illustrates the tendency of federal bodies rather to anarchy among the members, than to tyranny in the head. As a result, there should be little concern about the Union trying to abuse the power it is given. The essay is a good read, and a great refresher on some of the finer points of historical inquiry, but if you're in a hurry you can skip the history lesson and get right to the takeaway - a strong Union is always better than a weak confederacy.