The ratlines (German: Rattenlinien) were systems of escape routes used by German Nazis and other fascists to flee Europe from 1945 onwards in the aftermath of World War II. These routes mainly led toward havens in South America – particularly Argentina – in addition to Brazil, Chile, and Paraguay. Some escapees also settled at the various transfer points or used them to flee elsewhere.

Two primary routes from Germany to South America developed independently with their operators eventually collaborating; the first transferred through Spain and the second through Rome and Genoa. The ratlines were supported by some clergy of the Catholic Church, such as Austrian bishop Alois Hudal and Croatian priest Krunoslav Draganović, as well as some outlets of the International Committee of the Red Cross. The Nazis paid Argentine officials (starting c. 1943) to shield their agents, bolstering the rise of Juan Perón, whose regime set up additional ratlines through Scandinavia and Switzerland.

Starting in 1947, the United States utilized Draganović's network and an official at the International Refugee Organization to help Gestapo leader Klaus Barbie (who was in their custody in occupied Austria) flee to Bolivia. Details of the ratlines have continued to be released up to eight decades after WWII.

Two primary routes developed independently but their operators eventually collaborated. The first went from Germany to Spain, then Argentina; the second led from Germany to Rome, then Genoa, and finally South America. As many as 9,000 Nazi war criminals and their collaborators reportedly escaped to Argentina (up to 5,000), Brazil (up to 2,000), and Chile (up to 1,000). Some refugees immersed themselves in Latin America by pretending to be farmers and/or Catholic.

The origins of the first ratlines are connected to various developments in Vatican-Argentine relations before and during World War II. As early as 1942, the Vatican Secretary of State Cardinal Luigi Maglione – evidently at the behest of Pope Pius XII – contacted an ambassador of Argentina regarding that country's willingness to accept European Catholic immigrants in a timely manner, allowing them to live and work. Anton Weber, a German priest who headed the Roman branch of Saint Raphael's Society [de], travelled to Portugal with intentions to continue to Argentina, seemingly to lay the groundwork for Catholic immigration.

Some Catholic leaders accepted working with the Nazis to fight the common enemy of Bolshevism. By 1944, ratline activity centered in Francoist Spain was conducted to facilitate the escape of Nazis. Among the primary organizers were Charles Lescat, a French member of Action Française – an organization suppressed by Pope Pius XI and rehabilitated by Pius XII – and Pierre Daye, a Belgian with contacts in the Spanish government. Lescat and Daye were the first to flee Europe with the help of Argentine cardinal Antonio Caggiano.

By 1946, there were hundreds of war criminals in Spain, as well as thousands of former Nazis and fascists. According to U.S. Secretary of State James F. Byrnes, Vatican cooperation in turning over these "asylum-seekers" was "negligible". Unlike the Vatican emigration operation in Italy which centered on Vatican City, the Spanish ratlines – though fostered by the Vatican – were relatively independent of the Vatican Emigration Bureau's hierarchy.

Austrian Catholic bishop Alois Hudal, a Nazi sympathiser, was rector of the Pontificio Istituto Teutonico Santa Maria dell'Anima in Rome, a seminary for Austrian and German priests, and "Spiritual Director of the German People resident in Italy". After the end of the war in Italy, Hudal became active in ministering to German-speaking prisoners of war and internees who were being held in camps throughout Italy. In December 1944, the Allies allowed the Vatican to appoint a representative to visit the German-speaking civil internees in Italy, a job assigned to Hudal.