After being bitter enemies in both the War of Independence and the War of 1812, Britain and the United States took a long time to change their views of one another. Each remained skeptical of the other's intentions, whether the issue was commerce, Central and South America, Canada, Cuba, or a host of areas of potential dispute. Nevertheless, Anglo-American relations gradually improved, aided by a series of treaties negotiated in the 1840s and 1850s. By the time civil war broke out in the United States, the Government in Washington was reasonably certain that Britain would refrain from intervening, and declare itself neutral.
Looked at through British eyes, one of the most striking features of the United States during the 1840s was its unprecedented expansion. After becoming independent from Mexico in 1836, Texas was admitted to the Union in 1845. A year later, in a treaty between the United States and Great Britain, the dimensions of the Oregon territory were established, defining the boundaries of Oregon, Washington, Idaho, Montana, and Wyoming. At the same time, Britain agreed to relinquish its claim to the land between the 49th parallel and the Columbia River, thereby enabling the northern frontier to extend from Minnesota to the Rocky Mountains and the Pacific Ocean. Further enlarging the American land mass were the newly acquired territories of California, Arizona, and New Mexico, spoils from the war with Mexico. Thus, within four years, the United States had become a Pacific Ocean power in addition to dominating the North American shores of the Atlantic and the Gulf of Mexico.
The discovery of gold in California in 1849 kindled a desire in America to develop a route, and eventually a canal, across the isthmus of Central America. However, this was a region in which the British also had an interest, both diplomatic and commercial. Although generally considered more trouble than it was worth, given its poverty, continual internal strife, and distance from Europe, Britain nonetheless hesitated to withdraw from Central America because it would further encourage American expansion and perhaps military occupation, since Southerners were already eyeing Mexico and Central America as future states where slavery could flourish.
In their projections for connecting the eastern part of the country with the west, American entrepreneurs promoted the idea of creating a route across Central America through Nicaragua. At first, they were reconciled with steamers navigating the San Juan River and Lake Nicaragua and