The Lewis and Clark Expedition (1804-1806) explored the Louisiana Territory and routes for westward expansion to the Pacific. With the discoveries, the USA linked ownership claims to the Oregon area, which included the present-day states of Oregon, Washington, Idaho, and parts of Montana and Wyoming. West Florida (Alabama, Louisiana), disputed between Spain and the USA, was annexed by the USA in 1810, and East Florida was acquired by purchase in 1819. In parallel, Indiana (1816), Mississippi (1817), Illinois (1818), Alabama (1819), Maine (1820), Missouri (1821), Arkansas (1836), and Michigan (1837) were admitted to the Union. After around 35,000 American settlers in Texas declared their independence from Mexico, Texas became the 28th state admitted to the Union in 1845. A year later, the present north-western border of the USA was established when Great Britain recognised the area south of the 49th parallel as American. In parallel, Washington sought to buy large areas in the southwest, including California, from Mexico, but its government refused. The USA then provoked a border incident on the Rio Grande, which led to a declaration of war in 1846. In the peace treaty of 1848, Mexico had to relinquish the vast territory of today's states of California, Arizona, Nevada, Texas, Utah and parts of New Mexico, Colorado, and Wyoming in exchange for compensation of 15 million dollars. The expansion of the USA thus reached a peak between 1845 and 1849. In 1853, the USA acquired southern Arizona and New Mexico for the construction of a railway line to the Pacific. With the purchase of Alaska from Russia in 1867, the present continental borders of the USA were created.