This article means that the different states have agreed to engage in mutual defense. If one state is attacked, the others, in "a firm league of friendship," will help "assist" the state under attack, for whatever reason that attack occurs.
The Articles of Confederation created an inadequately weak bond between the states, treating them each as sovereign nations with open borders. It was much more like the European Union today than what we would call a modern nation state. As we can see from Article III, there is no legislation for common defense, no fundraising for an army, and nothing but the weakest of agreements to supply . . . something . . . if another state is attacked. The lack of specificity is alarming and "friendship" is a very flexible term.
The nation's founders quickly realized this document was not adequate to the nation-building goals they had in mind, which was why the Constitution was written.
The Articles of Confederation comprised the first constitution of the United...
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