The Federalist #3

Author - Jay
"It is of high importance to the peace of America that she observe the laws of nations towards all these powers, and to me, it appears evident that this will be more perfectly and punctually done by one national government than it could be either by thirteen separate States or by three or four distinct confederacies."

Overall, this is a pretty weak argument for the formation of a Union on the scale we have. I suggest that a few confederacies would be fairly adept at keeping each other in check in regards to their potential hostilities to nations outside of those neighboring confederacies. The treaties of close confederacies would have stronger treaties than distant nations would. This would inhibit unilateral hostility by one of the confederates. Jay goes on to expound quite the opposite view and it is unconvincing at best. His point would be nearer to the mark in light of each state being a sovereign nation.

"As to those just causes of war which proceed from direct and unlawful violence, it appears equally clear to me that one good national government affords vastly more security against dangers of that sort than can be derived from any other quarter."

That's a reasonable assumption. As an example, I believe that the West Coast would be under heavier threat from China today if the US wasn't a cohesive Union. It's highly possible that California would have come under the direct control of China a long time ago.