It was the ancient Persian empire my good sir <span />
Japan needed abundant natural resources to support both its population and its empire, something that the country lacked.
What I can infer from what Lewis and Clark saw when moving west from the Mississippi river is that the landscape along the river changed drasticallly. The forest receded, replaced first by the high prairie and then the shorter grass of the high plains. The temperatures became colder and as they continued their expedition by the river they realized that there was no water route to the pacific ocean, just more mountains.
