A: Once France fell in summer of 1940, and specifically Paris fell, refugees started streaming to the south of France. As Germany consolidated its control, they moved into Spain and Portugal to try to get out of Europe to get to America or somewhere else in the free world.
Many of these refugees were Jewish, and they were desperate to escape the horrors of the Nazis. As Arthur Koestler said [in a 1941 book], "Lisbon was the bottleneck of Europe." It was the last chance to get out.
[Koestler, a journalist, added that Lisbon was "the last open gate of a concentration camp extending over the greater part of the Continent's surface."]
Q: How easy was it to get out of Lisbon to a place like America?
A: It was very difficult. The leader of Portugal, António de Oliveira Salazar, saw the refugees as a hugely complicating factor. He's trying to maintain Portugal’s neutrality, and feared a German invasion of Portugal. The Portuguese also feared a proxy invasion by Spain, particularly during 1940 and 1941.
He had to juggle a lot of balls in the air. There was a hugely important complicating factor in the question of tungsten, a rare ore that’s mined in the northeast of Portugal and is absolutely vital to arms industries. Without tungsten, it's impossible to produce weapons. So you can see where this is going.
This was the key issue between the allies, the Germans and the Portuguese in the war: Would Portugal supply the Germans with tungsten? Salazar refused to stop selling to Germany.