Mr Manbar, 52, did the deals between 1990 and 1995. The court said that, eight years ago, he met Dr Majid Abasfur, head of Iran's "chemical warfare project" and committed himself to providing materials and training for chemical warfare. Mr Manbar, a former paratrooper, remained the owner of the Jerusalem Hapoel basketball team, but lived mostly in Europe from 1985.
"If I am guilty so are at least 200 people," said Mr Manbar. "Maybe I am guilty of naivete, but definitely not trying to harm the country."
The prosecutor said he supplied Iran with raw materials for the chemical weapons in 24 truckloads between 1990 and 1994. About a year-and-a-half ago he was secretly arrested when he landed in Israel.
Ignoring the fact that Mr Manbar's trial has not ended, Mr Netanyahu said yesterday: "This is a criminal who committed a terrible act the likes of which I cannot recall in the history of the state. He was arrested. He was tried. He was found guilty. I hope that he will pay a terrible price for his terrible deed." Amnon Zichroni, Mr Manbar's lawyer, described Mr Netanyahu's words as "vulgar interference in the judicial process".
Mr Manbar's perplexity about why he has been singled out for supplying Iran with arms is understandable. In the first two years of the Iran- Iraq war Israel is estimated to have sold Iran $100m of arms. When an Iranian F-4 Phantom jet defected to Saudi Arabia in 1984 it was found to contain parts shipped to Israel by the US.
Mr Manbar clearly got his timing wrong. At first he had supplied Israeli security with information about Iran, but they say he concealed some of his deals. Only in 1995 did they learn the true nature and extent of his business. He made a poor impact on the court. "His testimony was full of contradictions to say nothing of outright lies," said one of his judges.
The Iran-Iraq war saw the most sustained use of chemical weapons since the First World War. Most were used by Iraq. Iran said it had 50,000 casualties from gas, the fear of which demoralised the Iranian army in the latter stages of the war.
Although Israel was a major arms supplier to Iran during its war with Iraq, it has recently been putting pressure on America to stop Russia helping Iran develop a missile capable of hitting Israel. Mr Manbar evidently did not appreciate the switch in Israeli policy.
Given its losses to poison gas, it is likely that Iran's chemical weapons project is primarily aimed at acquiring a deterrent against Iraq.
In 1988 Iraq sent a veiled warning to Iran that it might put unconventional warheads on the missiles it was firing at Tehran. Iraq intended to send in fighter bombers first to smash windows to allow the poison gas to spread more easily. The Iraqi threat is believed to be why Iran accepted a ceasefire.