You can trace it to all the documents of union between the colonies the first one being...

"The Articles of Confederation of the United Colonies of New England"; May 19, 1643

Article 2 "The said United Colonies for themselves and their posterities do jointly and severally hereby enter into a firm and perpetual league of friendship and amity for offence and defence, mutual advice and succor upon all just occasions both for preserving and propagating the truth and liberties of the Gospel and for their own mutual safety and welfare."

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The Original Articles of Confederation were called....

"Articles of Confederation and Perpetual Union"; July 12, 1776.

and had stronger wording...

Article 2. "The said Colonies unite themselves so as never to be divided by any Act whatever, and hereby severally enter into a firm League of Friendship with each other, for their common Defence, the Security of their Liberties, and their mutual and general Welfare, binding the said Colonies to assist one another against all Force offered to or attacks made upon them or any of them, on Account of Religion, Sovereignty, Trade, or any other Pretence whatever."

You can see with this version it had to do with danger and that they would all promise to stay together to fight their enemy.

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David Upton