The United States of America (USA), commonly referred to as the United States (US), America, or simply the States, is a federal republic consisting of 50 states, 16 territories, a federal district, and various overseas extraterritorial jurisdictions. The 48 contiguous states and the federal district of Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. At 3.79 million square miles (9.83 million km2) in total and with around 316 million people, the United States is the fourth-largest country by total area and third largest by population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The geography and climate of the United States is also extremely diverse, and it is home to a wide variety of wildlife.
Paleo-indians migrated from Asia to what is now the US mainland around 15,000 years ago, with European colonization beginning in the 16th century. The United States emerged from 13 British colonies located along the Atlantic seaboard. Disputes between Great Britain and these colonies led to the American Revolution. On July 4, 1776, delegates from the 13 colonies unanimously issued the Declaration of Independence. The ensuing war ended in 1783 with the recognition of independence of the United States from the Kingdom of Great Britain, and was the first successful war of independence against a European colonial empire. The current Constitution was adopted on September 17, 1787. The first 10 amendments, collectively named the Bill of Rights, were ratified in 1791 and guarantee many fundamental civil rights and freedoms.
Human migration is movement by humans from one place to another, sometimes over long distances or in large groups. Historically this movement was nomadic, often causing significant conflict with the indigenous population and their displacement or cultural assimilation. Only a few nomadic people have retained this form of lifestyle in modern times. Migration has continued under the form of both voluntary migration within one's region, country, or beyond and involuntary migration (which includes the slave trade, trafficking in human beings and ethnic cleansing). People who migrate into a territory are called immigrants, while at the departure point they are called emigrants. Small populations migrating to develop a territory considered void of settlement depending on historical setting, circumstances and perspective are referred to as settlers or colonists, while populations displaced by immigration and colonization are called refugees. The rest of this article will cover migration in the sense of a "change of residence", rather than the temporary migrations of travel, tourism, pilgrimages, or the commute.
According to the International Organization for Migration's World Migration Report 2010, the number of international migrants was estimated at 220 million in 2013. If this number continues to grow at the same pace as during the last 20 years, it could reach 405 million by 2050. While some modern migration is a byproduct of wars (for example, emigration from Iraq and Bosnia to the US and UK), political conflicts (for example, some emigration from Zimbabwe to the UK), and natural disasters (for example, emigration from Montserrat to the UK following the eruption of the island's volcano), contemporary migration is predominantly economically motivated. In particular, there are wide disparities in the incomes that can be earned for similar work in different countries of the world. There are also, at any given time, some jobs in some high-wage countries for which there is a shortage of appropriately skilled or qualified citizens. Some countries (e.g., UK and Australia) operate points systems that give some lawful immigration visas to some non-citizens who are qualified for such shortage jobs. Non-citizens, therefore, have an economic incentive to obtain the necessary skills and qualifications in their own countries and then apply for, and migrate to take up, these job vacancies. International migration similarly motivated by economic disparities and opportunities occurs within the EU, where legal barriers to migration between member countries have been wholly or partially lifted. Countries with higher prevailing wage levels, such as France, Germany, Italy and the UK are net recipients of immigration from lower-wage member countries such as Greece, Hungary, Lithuania, Poland and Romania.