Long before the U.S. soccer team arrived in this balmy coastal city for the World Cup, another group of uniformed Americans came through. They turned this isolated part of northeast Brazil — the closest point in the Americas to Africa — into a World War II boomtown.

President Franklin Roosevelt dubbed Natal the "Trampoline to Victory," for keeping allied troops in Africa supplied. During part of the war, Natal was the busiest airport in the world, with flights taking off and landing every three minutes.

The U.S. team will play its first 2014 World Cup game Monday against Ghana in Natal, and thousands of American fans visited this weekend.

The last time so many Americans were here was from 1941 to 1947. When the U.S. Army closed its base, it left behind a local economic crisis and more than a few half-American babies.

Few outward signs of the World War II Brazilian-American connection can be seen today, but the U.S. military's presence was arguably the most important event for the city since it was founded, said Rostand Medeiros, a Natal native, historian and author.

"It was the principal event for Natal in the 20th century ... for the economy, for the population," Medeiros said through a translator.

The thousands of Americans coming in to watch their team likely know little of their country's past role here.

Jason Grammer didn't, even though his father probably handled many of the materials that flew out of this city. Herman Grammer, was a supply sergeant with the Fifth Army in World War II and was part of the allied invasions in North Africa and Italy.

"I had no idea," Jason Grammer said about Natal's U.S. connection. "My parents came down here on a cruise once. My dad mentioned something about Natal."

Grammer, 51, a health care technology professional from Piedmont, California, and his traveling crew of four friends chose to stay in Natal because they could easily travel to several matches in this part of the country.

"The war, I didn't know anything about that," he said.

Natal was chosen for the war effort because of its proximity to Africa. In the era of short-range flights, the distance and good weather in an equatorial city made it an ideal launching point for moving cargo and troops.

Earlier aviators — including Amelia Earhart and Charles Lindbergh — also used Natal.

The need to build roads, barracks and industry to support the thousands of troops — estimated at about 20,000 — changed Natal quickly. The prewar population of 40,000 doubled as rural Brazilians swarmed in for work.

The soldiers brought a casual approach to dress, style and attitude that fit in well with the low-income locals, Medeiros said.

They went to the beach, partied and some even got married. They brought new styles of music — the Glenn Miller Orchestra played the local USO — and new customs, with locals adopting the American pilots' use of the thumbs up sign for "all good."

They also romanced the locals, which sometimes caused problems.

"They left a lot of babies," Medeiros said, adding that if a woman had a child outside of marriage, she risked being kicked out of her family.

The Americans left soon after the war ended, pushed out by the new government that took power. Natal had planned for the Americans to stay another six or seven years, Medeiros said.

The departure meant lost investments that left the city in an economic funk for a decade. Since then, Natal's U.S. connections have faded, Medeiros said.

"Most of the people know the Americans were here, but they don't know the details. Those are only in the heads of the historians"