Secession is the act of withdrawing from an organization, union, or political entity. Typically there is a strong issue difference that drives the withdrawal.
The most notable example of successful secession in the modern era is the Declaration of Independence by the Thirteen Colonies from the British Crown in 1776. This event is often referred to as the American Revolution. More accurately this was a secession movement as opposed to a revolution. Revolutions seek to replace current governments, secession movements seek to separate from current governments.
Other secession movements include the case of the Southern states of the United States seceding to form the Confederate States of America. Less dramatically, new American states were commonly formed out of an older state as the United States grew, such as in the northeast (Maine created out of Massachusetts), the mid-Atlantic (Kentucky created out of Virginia) and then repeatedly in the western territories. The formation of such states are not typically considered secessionist because they were officially accepted by the parent state and the national government.
Local examples of secession also exist, such as Piedmont, California, which was part of Oakland before seceding from the latter in 1907 and the attempt of Staten Island to break away from New York City in the late-1980s and early 1990s. San Fernando Valley recently lost a vote to separate from Los Angeles County but has seen an increased attention to its infrastructure needs. Several cities in Vermont including Killington are currently exploring a secession request to allow them to join New Hampshire over claims that they are not getting adequate return of state resources from their state tax contributions.
There have been other modern secessionist movements to create new states. There was a short-lived effort to create a Jefferson State out of counties in southern Oregon and northern California in 1941, in part motivated by requests for better roads, but it was quickly shelved by the outbreak of World War II. Advocates in the upper peninsula of Michigan, with off and on intensity, have called for it to become a separate 51st state. There have been calls for formation of Cascadia in the Pacific Northwest. A less ambitious plan would create a new state from Washington State east of the Cascade Mountains, along with the northern Idaho, northwestern Montana, and possibly northeastern Oregon. It would be centered on Spokane, Washington (the largest city in the region), and called "Columbia" after the Columbia River.
There are also web sites currently advocating a separate California nation, and independent nation of Hawaii as well as other sections of the United States.
Many articles after the 2004 Presidential election questioned whether the so-called "blue" and "red" states can continue to co-exist or ever reconcile or if they might be drifting toward irreparable policy differences and social conflict and possible future separation. Alternatively it is possible the political conflict may result in gradual diminution of the federal government- for lack of a true national consensus - and perhaps a greater emphasis on state rights to permit them to chart more of their own domestic agendas while maintaining the federal union for a more limited set of national actions than undertaken today and for international purposes.