The armed forces of many nations have, at one time or another, used foreign volunteers who are motivated by political, ideological or other considerations to join a foreign army. These may be formed into units of a given nationality or may be formed into mixed nationality foreign units. Sometimes foreign volunteers were or are incorporated into ordinary units. The practice has a long history, dating back at least as far as the Roman Empire, which recruited non-citizens into Auxiliary units on the promise of them receiving Roman citizenship for themselves and their descendants at the end of their service.