Article III of the Articles of Confederation is a promise made by the various states to defend one another and to be allies. What it is saying is that they will all help each other if any of them is attacked by an outside power.
When thinking about this, it is important to remember that the states, under the Articles of Confederation, were almost like independent countries. As Article II says, each state retains its sovereignty, which means that it acknowledges no government that is higher than it. Because the states saw themselves as sovereign, it was important for them to ally with one another so that (at least in the case of defense) they could act like one country.