A 1937 dispute between Czechoslovakia and Portugal over the sale of 600 machine guns led to a break in diplomatic relations between the two countries that lasted nearly 37 years.

[5][6] Against this backdrop, Portugal had aligned itself with the Nationalist forces in Spain's ongoing Civil War, which were battling the Soviet-backed Republican faction.

[8] Meanwhile, in early 1937, several Czechoslovak citizens had been expelled from Portugal after the Portuguese government accused them of being engaged in unspecified illicit activities on behalf of Spanish Republican forces.

[12][9] The request for assurance personally offended Portuguese prime minister António de Oliveira Salazar and relations between Czechoslovakia and Portugal quickly deteriorated.

Naval Institute's Proceedings, "Portugal defended her action on the ground that the refusal of the Prague government was determined not by its own arms needs or nonintervention pledges but by instructions from Moscow".