Colonial Empires spread around the world, like the British Empire or the French and Spanish Empires, aren't the only sort.

The continental United States, "sea to shining sea", is surely every bit as much an Empire as, for example, the Austrian Empire or the German Empire, or the Chinese or Russian Empires - it's just that, as was the case with those, the conquered territories incorporated were for the most part contiguous rather than overseas. That's what was surely meant by "manifest destiny"?