"Never have we sought annexation of our allies, nor even of our enemies."

Then how come the United States stretches across a whole annexed continent? The fact that the occupied teritory was contiguous rather than scattered around the place shouldn't disguise the fact that the United States is essentially an Empire. After all, the same could be said of most Empires - Roman, Chinese, Russian, Austrian...

By this logic, Canada, Brazil and Australia are also Empires, since the territory of their countries consists of land annexed from indigenous peoples.

Fact is, the annexation of North American land in the 18th and 19th Centuries represented competition between the newly-created United States and the colonial powers such as England, France, Spain and Russia who all held land in what is now the US. Whether or not these powers had legitimate claims to the territory held is another question entirely.