Oct. 4, 2013 

WASHINGTON -- As U.S. veterans of the October 1993 "Black Hawk Down" battle in Somalia honor their fallen this week, others also remember it as the 20th anniversary of America's first blow from al Qaeda -- even though the U.S. didn't know it at the time.

The street battles that ensued on Oct. 3 and 4, 1993, involved U.S. Army Rangers and commandos from the 1st Special Forces Operational Detachment-Delta, known as "Delta Force." During a successful mission to capture lieutenants of Somali warlord Mohamed Farah Aideed, hundreds of Somali gunmen engaged the U.S. teams and killed 18 soldiers, wounded 73 others and shot down two Army Black Hawk choppers.

America's subsequent hasty exit from that mission, which was originally to support United Nations humanitarian operations in Mogadishu, emboldened a little-known terrorist leader at the time named Osama bin Laden, who boasted that the U.S. superpower was weak for withdrawing after losing G.I.s in "minor battles" there.

"You left [Somalia] carrying disappointment, humiliation, defeat and your dead with you," bin Laden taunted in his 1996 fatwa against America.

What bin Laden didn't say in 1996 was that his henchmen had a hand in training and equipping the Somali militiamen who inflicted the worst day of casualties in the history of U.S. Special Operations Forces. Deadly al Qaeda attacks in the Horn of Africa against U.S. targets in 1996, 1998 and 2000 followed, leading up to 9/11.

"It is true that al Qaeda was emboldened by 1993 – it was their first successful attack on us and we were unaware of bin Laden's involvement until later," former Sen. Bob Kerrey, who served on the 9/11 Commission, told ABC News on Thursday.

"They coached the [Somali militia] rocket-propelled grenade guys to aim for the tail rotors of U.S. Black Hawks," Mark Bowden, author of the best-selling book "Black Hawk Down," told ABC News.

WATCH: Former Army Ranger Revisits 'Black Hawk Down,' 20 Years Later

Bowden argued that al Qaeda's overall role wasn't extensive or decisive in the battle, and their support to the Somali warlord "wasn't a coherent plot" in the way their future attacks would be.

But it did foreshadow a long fight against the extremist group.

The 9/11 Commission disclosed that the CIA did not learn until 1997 that bin Laden had sent top military experts to Somalia almost a year before the Black Hawk Down battle to aid Aideed.

RELATED: Black Hawk Shudown, Politics Plague New Mogadishu Honor

Al Qaeda military chief Mohammed Atef, an Egyptian known as "Abu Hafs," was sent by bin Laden to Mogadishu, where he offered to help Aideed fight U.S. and U.N. forces, according to FBI files. In 1998, after two U.S. embassies were struck by suicide bombers in Kenya and Tanzania, the FBI interviewed Al Qaeda operative Mohammad Sadiq Odeh in Nairobi, who admitted he was among the men al Qaeda shura council member Saif el Adel -- also a former Egyptian military officer -- had ordered to Somalia before the attack on the Rangers and Delta operators.

"Odeh stated that his mission in Somalia was to train some of the tribes fighting and to provide food and money," FBI Agent Dan Coleman wrote in a report then.

Coleman also interviewed an active-duty U.S. soldier, Army Sgt. Ali Mohamed, a Special Forces Middle East expert at Fort Bragg who later pled guilty to scouting the U.S. embassies for bin Laden before they were bombed, in addition to training al Qaeda fighters in Afghanistan.

"Mohamed also advised that he was in Somalia during the United States intervention overseas and knew that bin Laden's people were responsible for the killing of United States soldiers in Somalia," Coleman reported back to the FBI in 1998.

"A small group of Afghanistan [mujahideen] veterans undertook a number of skillful operations against the Americans in Somalia," an al Qaeda internal history published a decade ago stated. "When the valiant soldiers of Islam came to them with the rod of Moses and the mujahideen poured their fire on them, the Americans withdrew from Somalia in an unexpected haste."

Federal terrorism prosecutor Patrick J. Fitzgerald obtained a sealed indictment in 1998 against bin Laden which stated that, "members of al Qaeda participated with Somali tribesmen in an attack on United States military personnel serving in Somalia."

"We can't be certain that he killed those 18 Rangers in the battle of Mogadishu on October 3rd and 4th," Fitzgerald later testified at a 9/11 Commission hearing, "but he celebrated it afterwards, and he sent trainers down there, and he sent weapons down there."

Kerrey, a Navy SEAL recipient of the Medal of Honor in Vietnam, recalled visiting the wounded from Mogadishu at Walter Reed Army Medical Center in late 1993, never imagining the fight that the incident foreshadowed.

Not only did the U.S. not know about al Qaeda until years afterward, but policymakers have -- even since 9/11 -- often "underestimated" the terror network's resiliency up until the present with its possible role in Nairobi, Kenya's deadly Westgate mall attack by Somali al-Shabaab gunmen last week, Kerrey said.

Bin Laden also said in his 1996 speech that the U.S. pulled out of Beirut, Lebanon, 30 years ago after the U.S. Marine compound there was bombed, and Shabaab and al Qaeda may be banking on Kenyan public opinion to similarly turn on the current Somalia intervention, said another former Special Operations commander.

"When we were in Iraq, there were times when the insurgency repeatedly was trying to achieve the 'Beirut effect' -- those were their words -- to kill enough Americans in multiple attacks to make us pull out," retired Army Lt. Col. James Gavrilis, who led Special Mission Unit operations, told ABC News. "It's not fabricated, it's a real pattern."

READ MORE: Black Hawk Down, An American War Story (Philadelphia Inquirer)

ABC News' Lee Ferran contributed to this report.