Answer:
The westward expansion began in 1803 with Louisiana Purchase, it was fuelled by the Oregon trail, Gold Rush and a firm belief in 'manifest destiny'.
The westward expansion ended on February 14, 1912 when Arizona was admitted to the union, it was the last among 48 contiguous states of US.
It began soon after the establishment of the first colonial settlements along the Atlantic coast. Economic opportunity and geography influenced this movement. As land was considered to be a sign of power, the expansion of the US territories made it a stronger nation,
The expansion transformed the size and function of the county, it boosted the economy as more land was brought under cultivation and new factories were established. Increased production in US increased trade with other countries. United states became a confident and wealthy nation.
One negative impact of the westward expansion was that native Americans were expelled from their lands and had to live on small reservations, it had devastating affect on them. It wiped out many tribes, caused tension, wars and cultural divide.