John Jay writes in Paper No. 5 of a letter that Queen Ann wrote to the Scottish Parliament in 1706 in favor of the union of England and Scotland:
An entire and perfect Union will be the solid foundation of lasting peace: It will secure your religion, liberty, and property, remove the animosities amongst yourselves, and the jealousies and differences betwixt our two kingdoms. It must encrease your strength, riches, and trade: and by this Union the whole Island, being joined in affection and free from all apprehensions of different interest, will be enabled to resist all its enemies.
There's a sales job if there ever was one. Buy united elixir! It will remove blemishes, relieve tooth pain, improve the insulation of your home, and generally shower riches upon you - or it certainly will for the salesperson since she will have your revenue. However hyperbolic Queen Ann's promises, they worked! Scotland and England created a formal political union in 1707. It took only 150 years for the Scots to start campaigning for Home Rule. An independence referendum was voted on as recently as 2014 and almost 45% of Scots voted for independence. When the UK voted on Brexit, 62% of Scots voted to remain in the EU in contrast to the 48% of British citizens overall. The First Minister of Scotland claimed this as a justification for another independence referendum, but the Prime Minister declined to put it to a vote so soon after the last one.
Jay adds to the arguments he already made by using the example of the history of Great Britain to support why a united nation is preferable to a divided one, The disagreements between the three nations that previously comprised Britain, he claims, kept them in a nearly perpetual state of war centuries, weakening their ability to defend themselves from the attacks by foreign enemies. He proposes that multiple nations on the American landmass would create policy and border disputes. He cautions that these nations will not necessarily be equal in their in their strengths, as if to say, if you welcome war don't be so certain that you won't lose.
Separate nations would also have separate commercial interests, and their commerce with other countries would be regulated by separate treaties. So adjacent nations could end up taking opposing sides in a dispute with a third party, risking the dangers of war with each other.
I have always found it interesting when Americans argue - why should we care what other nations think? Clearly the founders of our government had a different idea. They understood that power is defined by the ability to defend oneself and to support oneself via commerce, and those abilities are dependent upon our relationships with other nations.