The commander of the Bay of Pigs invasion, José Perez San Roman, kneeled and kissed the sand with joy when he landed at Playa Girón on the south coast of Cuba. Two days later, his 1,500 men had been thoroughly defeated.

“We are out of ammo and fighting on the beach. Please send help,’’ San Roman radioed his CIA advisors. Then, this final transmission: “I have nothing to fight with. Am taking to the woods.’’

The most direct and powerful U.S. bid to topple Fidel Castro began amid rosy optimism 50 years ago on April 17. It ended in disaster April 19.

President John F. Kennedy and the CIA were forever seared by the historic failure. Castro became the Caribbean David who defeated the Goliath to the north. His grip on the reins of power grew ever more powerful. And 18 months later, the Cuban Missile Crisis brought the world to the brink of nuclear war.

Castro branded the captured Brigade 2506 invaders as “mercenaries” and demanded ransoms for their release – from $500,000 for San Roman and each of the two other invasion leaders, down to $25,000 for the foot soldiers.

Yet survivors of the Brigade’s assault today recall the three days of fighting and 20 months in Castro’s appalling prisons as a heroic moment for them and a luminous moment in Cuba’s struggle for democracy.

The brigadistas landed in the predawn darkness of April 17, 1961 — five frogmen and one CIA case officer, Grayston Lynch, who were to plant lights on the beaches to guide ashore the rest of the amphibious assault force.

The exile fighters were to follow and seize a 40-mile long front on the eastern shore of the Bay of Pigs — from Playa Larga in the north to Playa Girón in the middle and Caleta Verde to the south.

In the first hours, the invasion seemed to go well.

“We repelled three attacks during daylight, including one in the afternoon [by] more than 1,000 militia and army” troops, wrote Erneido Oliva, head of Playa Larga operations and the landing force’s No. 2 military commander.

Brigade paratroopers seized two key roads to the beachheads — narrow causeways built over the largest swamp in the Caribbean, the Cienaga de Zapata. The infantry secured an airstrip needed to resupply the invaders and fly in a civilian “government’’ that would call for international recognition.

Six Brigade B-26 planes dropped 250-pound bombs on the first and last vehicles of a Castro police and militia convoy caught on one causeway, then raked the rest with the eight 50-caliber machine guns mounted on their noses. Cuba later reported 1,800 people were killed or wounded in that one battle alone.

“For me, those 15-20 minutes seemed like an hour. For the people down there, it must have seemed like an eternity,’’ recalled Gustavo Villoldo, who flew aboard one of the B-26s.

In another bloody battle, Brigade fighters controlled the critical San Blas causeway through three days under nearly constant pounding by Castro’s long-range artillery and waves of attacks by ground troops and Soviet-made T-34 tanks.

Mortar crewman Mario Martinez-Malo recalled that at one point, the number of civilians and Castro militias captured in just one area was more than twice the number of invaders.

On the third day of fighting, he fired 405 shells at an advancing militia column on a causeway, Martinez-Malo added, without the time to move away and cover his ears in between shells. “By the end, I was deaf.”