On the outbreak of the Spanish Civil War the Portuguese government of Antonio Salazar immediately supported the Nationalists in the struggle against the Popular Front government in Spain. Salazar feared that if the Republicans won the war his own authoritarian government would be under threat.

Salazar, concerned about the effect the events in Spain would have on his country, established a new militia that could serve as an auxiliary police. This new police force arrested dissidents and removed politically unreliable people from educational and governmental institutions.

Leaders of the Nationalist Army were allowed to negotiate with representatives from Nazi Germany in Portugal. After the signing of the Non-Intervention Agreement in September 1936, Salazar agreed that Germany could disguise the aid that it was giving by sending men, planes, tanks, and munitions via Lisbon. Salazar's police also arrested supporters of the Popular Front government living in Portugal. He also sealed off the Portuguese frontier to Republicans.

Although he came under considerable pressure from Britain and France, Salazar refused to allow international observers to be stationed on the Portugal-Spain border. Officially he claimed that it would be a violation of Portugal sovereignty while in reality he did not want the world to know about the large amounts of military aid that was crossing into Spain.