In fact the Monroe Doctrine, imposing on Europe a self-denying policy of
non-colonial expansion toward the west, provided for the United States
the medium, if she wished to use it, for her own expansion in territory
and in influence. But for a time there was no need of additional
territory for that already hers stretched from the Atlantic to the Rocky
Mountains, two-thirds of the way from ocean to ocean. Her population was
growing fast. But four millions at the time of the Revolution, there
were thirteen millions in 1830, and of these nearly a third were already
across the Appalachian range and were constantly pressing on towards new
lands in the South and West. The Monroe Doctrine was the first definite
notice given to Europe of America's preconceived "destiny," but the
earlier realization of that destiny took place on lines of expansion
within her own boundaries. To this there could be no governmental
objection, whether by Great Britain or any other nation.

But when in the decade 1840 to 1850, the United States, to the view of
British statesmen, suddenly startled the world by entering upon a policy
of further territorial expansion, forsaking her peaceful progress and
turning toward war, there was a quick determination on a line of British
policy as regards the American advance. The first intimation of the new
American policy came in relation to the State of Texas which had
revolted from Mexico in 1836, and whose independence had been generally
recognized by 1842. To this new state Britain sent diplomatic and
consular agents and these reported two factions among the people--one
seeking admission to the American Union, one desiring the maintenance
of independence.

In 1841 Aberdeen had sent Lord Ashburton to America with instructions to
secure, if possible, a settlement of all matters in dispute. Here was a