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Immigration refers to the permanent resettlement of an individual from one country to another, either with legal authorization or without. Travel refers to shorter stays that are intended to be temporary. Both immigration and travel are subject to a variety of restrictions in countries throughout the world, and immigration has often been a source of social and political controversy. The government’s ability to restrict entry is rooted in the core principle of national sovereignty: that every nation has a basic right to protect itself by controlling its borders.