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Frail but determined, Pope John Paul II (search) arrived in Croatia on Thursday to begin his landmark 100th pilgrimage: a five-day visit to this Roman Catholic stronghold in the Balkans.

The 83-year-old pontiff's plane landed at an Adriatic island airport, where the ex-Yugoslav republic's president and top church leaders waited to welcome him. The pope was to be spirited by high-speed catamaran across a bay to Rijeka's harbor to greet pilgrims gathering on the docks.

Earlier, as he boarded the Alitalia flight in Rome, John Paul had difficulty walking and had to be helped by aides onto a special lift he uses to get on and off aircraft. The Italian airline presented the pope with a special cake in honor of his 100th trip.

John Paul's visit, which gets into full swing Friday with a visit to the war-battered southern coastal resort of Dubrovnik (search), will test anew his ability to overcome his advanced age, Parkinson's disease (search), hip and knee troubles and balmy summer weather.

The pope didn't hide his affection for this overwhelmingly Catholic country, offering Croats a blessing "with all my heart" on the eve of his departure from Rome and asking for their prayers. They were quick to answer with deep appreciation for being chosen as the venue for the historic trip.

"It shows how much the Holy Father cherishes Croatia, its church and its people," said Ivan Devcic, bishop of the Adriatic port city of Rijeka, which will serve as a base for the pope and his entourage.

About 80 percent of Croatia's 4.5 million people are Roman Catholics (search), and the Vatican was among the first to recognize the country's statehood in January 1992, six months after it declared independence from Yugoslavia.

For most Croats, the pope is the highest moral authority. Half a million faithful are expected to attend papal Masses in Rijeka, Dubrovnik, the southern coastal city of Zadar and the eastern cities of Osijek and Djakovo.

Photographs of John Paul are in shop windows and on billboards all over Croatia. Schools will be closed in honor of his visit, and many Croats intend to skip work to see him. Security was tight for the pope's arrival, with automobile traffic restricted in the cities awaiting him, but trains were available free of charge to pilgrims.

"The pope will bring us spiritual peace, and we need it," said Ana Brnabic, a 52-year-old worker.

John Paul first came here in 1994, just three years after a devastating war with the local Serbs who rebelled against Croatia's declaration of independence. He returned in 1998 to preach postwar reconciliation and tolerance.

This time, the pope will find a country caught between its desire to finally join the West and the lingering nationalism that threatens to keep it isolated.

Even though the fighting ended in 1995, and a pro-Western coalition government took power three years ago, nationalist sentiment remains strong.

War veterans and many ordinary Croats oppose the U.N. war crimes tribunal's prosecutions of Croat fighters whom they hail as war heroes. Minority Serbs, who fled the country in 1995 when it retook areas the rebels seized in 1991, are not quite welcome back.

Some youngsters carry memorabilia of Croatia's World War II Nazi puppet state. The nationalist party founded by the late President Franjo Tudjman enjoys 25 percent support and could even win elections this fall.

Wading gingerly into the fray, John Paul is expected to call anew on Croats to pursue tolerance, reconciliation and coexistence.

Peter Kuzmic, dean of a theological seminary in Osijek, said the pope is coming "out of great concern" over right-wing, antidemocratic elements still present here.

But John Paul's visit is mainly intended to show his support for Croatia's persistent faith and its efforts to become a part of the European mainstream. The pope often has encouraged Croatia's efforts to join the European Union, a prize some leaders hope to win as early as 2007.

With the theme "Family -- the path of the church and the nation," the pope also will emphasize the importance of family ties. In Dubrovnik, Croatia's tourist pearl heavily shelled by Serbs in 1991, the pontiff will beatify Marija Petkovic, a Croatian nun who died in Rome in 1966 after founding a religious order and devoting her life to children. Petkovic will be the first Croatian woman to be beatified, which is the last step before possible sainthood.

At Saturday's stop in the eastern city of Osijek -- just six miles from predominantly Orthodox Serbia -- the pope will meet with Orthodox church leaders in an ongoing effort to erase a millennium of tension and mistrust between Christianity's two quarreling sisters.