SEOUL — On the count of “hana, dul, set,” a dozen men hoisted a hefty, drab green metal beam above their heads and into its slot, one of the first links in what — two hours later — would become a 69-foot bridge on the bank of the Han River.

That bridge spanned a 59-foot hole in the dirt, dug to mimic a section of a bridge blown up during battle. But during a real attack, the temporary bridge would be a critical route for getting convoys across the water, one 60-ton tank at a time.

Fourteen U.S. Navy Seabees and 52 South Korean soldiers and sailors built the bridge on a chilly, overcast Thursday morning as part of this week’s U.S.-led Key Resolve/Foal Eagle military exercise.

“It’s kind of like an Erector Set,” said Seabee Lt. j.g. Chris Archer of the Navy Mobile Construction Battalion 5. “It’s putting the right pieces in the right order, like a puzzle.”

Knowing how to put together this kind of “Erector Set” is especially important in South Korea, whose capital city, Seoul, is split by the Han River. About two dozen bridges connect the northern and southern halves of the city. During the Korean War, Seoul residents were stranded in the war-torn city after U.S. and South Korean forces blew up its handful of bridges to keep North Korea from advancing farther south.

Archer said a possible attack on Seoul’s bridges by North Korea is a “big reason” the forces practice bridge building.

“If a case like that should happen, there’s a real good possibility that that’s exactly what we would be doing in that situation,” he said.

The Seabees, part of a California-based unit in Chinhae for a six-month deployment, said they practice building bridges several times a year.

South Koreans and Americans chanted “safety first” in Korean and then English as they lugged the 100- to 600-pound pieces of the bridge to the construction site.

“It’s heavy, but it goes together pretty easy,” said Petty Officer 1st Class Justin Hilton.

The South Koreans and Americans communicated mostly through hand gestures and tugs on their uniforms, talking through interpreters on occasion.

Petty Officer 3rd Class Mihyon Cha, who emigrated to the United States from South Korea when she was 6 years old, said the South Koreans led Thursday’s bridge-building exercise.

The two forces trained together in the days before the exercise. Cha said that during breaks, she and the South Koreans compared their countries and their militaries.

“It’s fascinating coming back to my homeland,” she said. “I’m not fighting for them, but I’m helping them defend their country.”

About two dozen South Koreans demonstrated against the bridge-building exercise in a nearby field, guarded by several dozen riot police.

Protester Byeon Yeon-sik said the exercise was “aggressive” and could undo the peace-building efforts of last month’s New York Philharmonic concert in Pyongyang, which marked the largest group of Americans to visit North Korea since the Korean War.