What was the war that had just ended that allowed the United States to call the 13 colonies land their own?
The American Revolutionary War
The American revolution was mainly fought by American colonists against Britain in order to gain their independence because they wanted more freedoms.
The American Revolutionary War is arguably a civil war caused by the ham-fisted dealings of the British Government.
Granted the British were new at the creation of the "Dominions" like Canada and Australia that they made subsequently. The War was mostly white Europeans (many British) fighting white Europeans and it was a disagreement about governance. This sounds very much like a civil war. Loyalists to the King went North and their descendants helped to create Canada. They were quite distrustful of Americans.
Further fighting was needed to open up ways to the West and for the United Stated to solidify their power base. After the War of 1812 the British stopped supporting the Indians against the Americans.
The British, the Americans, and the Canadians were able to manage their conflicts after the war of 1812. Relative peace and stability in Europe helped things along. Similarities, like language and common culture assisted.