FACTBOX: Who is Pope Benedict?

(Reuters) - Pope Benedict, who begins his first trip to the United States as pontiff on Tuesday, is only the third head of the Roman Catholic Church to visit the country.

Here are a few facts about the pontiff:

EARLY LIFE:

* Joseph Ratzinger was born in Marktl am Inn, Bavaria, Germany, on April 16, 1927. The son of a police chief, he spent most of his childhood in Traunstein, where he attended secondary school.

* During the early 1940's, Ratzinger was briefly a member of the Hitler Youth. A 1999 article in the National Catholic Reporter said Ratzinger was an assistant on an anti-aircraft battery guarding a BMW plant in 1943, then sent to the Austria-Hungary border to erect tank traps. After returning to Bavaria, he deserted. At the end of World War Two, he was an American prisoner of war.

RELIGIOUS BACKGROUND:

* From 1946 to 1951, he studied at theological college in Freising and then at the University of Munich before being ordained as a priest. He received a doctorate in theology in 1957 and became a professor at Freising college in 1958.

* Ratzinger was a liberal theological adviser at the Second Vatican Council, but became more conservative after the 1968 student movement prompted him to defend the faith against secularism.

* He was archbishop of Munich before taking over the Congregation for the Doctrine of the Faith in 1981, becoming the Roman Catholic Church's chief ideologue.

* In 2000, he branded other Christian churches as deficient -- shocking Anglicans, Lutherans and other Protestants who had been in ecumenical dialogue with Rome for years.

BENEDICT XVI:

* Ratzinger succeeded Pope John Paul II in April 2005. His first trip abroad as pontiff was to his homeland, Germany, in August 2005.

* Last month Vatican and Muslim leaders agreed to establish a permanent official dialogue to improve often difficult relations and heal wounds still open from a controversial papal speech in 2006.

-- Catholic-Muslim relations nosedived in 2006 after Benedict delivered a lecture in Regensburg, Germany, taken by Muslims as implying that Islam was violent and irrational.

-- Although Benedict repeatedly expressed regret for the reaction to his speech, he stopped short of a clear apology sought by Muslims.

(Writing by David Cutler, London Editorial Reference Unit, editing by Patricia Zengerle)