The operation allegedly began before the US-backed crackdown.
The case, set to open on Tuesday has all the elements of an international spy thriller: an ally of the United States in the fight against drugs in Peru stands accused of arming Washington's enemy in the world's cocaine capital.
And investigators say evidence points to CIA support of Montesinos, but they say agents have refused to answer questions about the matter.
The trial of the man who ran Peru from the shadows for a decade as ex-President Alberto Fujimori's right-hand man takes place in a courtroom on the naval base where Montesinos has been held since his arrest in June 2001.
Montesinos, who sparked an unprecedented corruption scandal that brought down Fujimori in 2000, has already been convicted on four lesser corruption charges. He denies smuggling rifles to the Revolutionary Armed Forces of Colombia, or FARC.
But his repeated attempts to get the case delayed show "he fears he'll be convicted," said Ronald Gamarra, a state attorney investigating Montesinos on a host of charges. Prosecutors are seeking the maximum 20-year sentence.
"It was a three-way deal. The cash came from (jailed Brazilian drug kingpin) Fernandinho, the Peruvians bought the guns, cheaply, and the FARC brought the drugs."
Jane Holligan, author
'Very important case'
Interior Minister Fernando Rospigliosi told Reuters: "This is a very, very important case because it shows how both Montesinos and Fujimori were involved in dealings with a terrorist and drug-trafficking group like the FARC.
"There is abundant evidence of the participation of Montesinos, which could bring a very heavy sentence."
Fujimori himself is in self-imposed exile in Japan, and Peru is seeking his extradition to face corruption and murder charges. The former president denies the charges.
Montesinos, 58, has remained silent so far in court to protest his jail conditions. His lawyer says he is now ill.
In a news conference in August 2000, in which Montesinos made an unusual public appearance, Fujimori announced Peru's security services had smashed a major international arms smuggling ring in what he code-named "Operation Siberia."
But prosecutors believe it was Montesinos who was the mastermind behind the deal in which 10,000 AK-47s were procured from Jordan and parachuted in 1999 to jungle-based FARC rebels - key targets of Washington's drug-busting "Plan Colombia" that was launched in 2000 and has provided about $2 billion in aid to date.
In an interview with El Comercio newspaper on Sunday, Gamarra said: "In the case of arms trafficking to FARC guerrillas, Montesinos appears to have counted on the support of the CIA. We do not have any hard proof of that, but we do have various indications that would prove this relationship."
"This is a very, very important case because it shows how both Montesinos and Fujimori were involved in dealings with a terrorist and drug-trafficking group like the FARC. There is abundant evidence of the participation of Montesinos, which could bring a very heavy sentence."
Fernando Rospigliosi, Interior Minister
He said Montesinos, who has been linked to the CIA since he was in the military in the 1970s, would have needed such support to pull off the deal.
A CIA spokeswoman in Washington declined to comment, saying only, "It's a matter before the courts."
Fujimori told the August news conference - which came just a month before a video showing Montesinos bribing a lawmaker was aired on television, igniting the scandal that toppled the government - that the guns were bought with drug money.
Jane Holligan, co-author of a book on Montesinos' rise and fall, said: "It was a three-way deal. The cash came from (jailed Brazilian drug kingpin) Fernandinho, the Peruvians bought the guns, cheaply, and the FARC brought the drugs."
Sarkis Soghanalian, an international arms dealer who is accused of procuring the guns, and his colleague Charles Acelor, say in official testimony that Montesinos was the brains of the plan, Jose Ugaz, a former state attorney investigating the spy chief, told Reuters.
"It seems a pretty solid case to me," he added.
Prosecutors say that even if he is convicted, Montesinos may walk free within a few years if the Supreme Court upholds a ruling allowing inmates to be released after serving less than one-third of their sentence.
Montesinos faces Peru's maximum 30-year term on other charges, including responsibility for 25 death-squad killings,
However sentences in Peru are not consecutive.