American's desire to expand westward intensified after the economic
instability resulting from the Panic of 1819.  The 1840s saw a rapid
migration of pioneers to the Oregon and California territories
(particularly California, during the Gold Rush of 1849) .  The end of the
Mexican War in 1848 facilitated the acquisition of western Indian lands,
and prices dropped rapidly.  The Indians, ill-equipped at first to combat
the influx of people (in this illustration from the 1850s no Indian
carries a rifle), were scattered from their territories in large numbers,
leaving the plains open for massive white settlement.