Douglas was the most thoroughgoing of expansionists and would acknowledge no northern boundary on this side of the North Pole, no southern boundary nearer than Panama.

He regarded the United States, with its great principle of local autonomy, as fitted to become eventually the United States of the whole world, while he held it to be an immediate duty to make it the United States of North America.

As the son-in-law of a Southern planter in North Carolina, and as the father of sons who inherited slave property, Douglas, although born in Vermont, knew the South as did no other Northern statesman.

He knew also the institution of slavery at first hand.

As a pronounced expansionist and as the congressional leader in all matters pertaining to the Territories, he acquired detailed information as to the qualities of these new possessions, and he spoke, therefore, with a good degree of authority when he said, "If there was one inch of territory in the whole of our acquisitions from Mexico where slavery could exist, it was in the valleys of the Sacramento and the San Joaquin." But this region was at once preempted for freedom upon the discovery of gold.
Douglas did not admit that even the whole of Texas would remain dedicated to slavery.