Omar Khyam, 24, of Crawley, West Sussex, is the first of seven Britons accused of the plot to give evidence in their trial.
He admitted travelling to Pakistan in 2000 for guerrilla training in weapons and reconnaissance tactics.
Khyam and six other men deny conspiring with a Canadian to cause explosions.
The defendants were arrested in March 2004 when fertiliser was found stored in a west London depot.
It is alleged they discussed bombing targets including pubs and nightclubs.
When asked about his reaction to the attacks on the World Trade Center Khyam said: "I was happy. America was, and still is, the greatest enemy of Islam.
"They put up puppet regimes in Muslim countries like Saudi Arabia, Jordan and Egypt.
"I was happy that America had been hit because of what it represented against the Muslims, but obviously 3,000 people died so there were mixed feelings."
He added that, after consulting with Islamic leaders, he later thought the attacks to be a tactical mistake.
Khyam also said he went to Pakistan for training with a group of mujahideen fighting in Kashmir.
"They taught me everything I needed for guerrilla warfare in Kashmir," he said.
"AK47s, pistols, RPGs, sniper rifles, climbing and crawling techniques, reconnaissance and light machine guns."
On a subsequent visit to Pakistan, Khyam had travelled across the border to Afghanistan to find out how the country worked under the Taleban.
"They were amazing people," he said.
"They loved Allah very much. This is how an Islamic state should be."
He also described the war in Iraq as "a war against Islam".
Earlier, Khyam told the court that in 1998 he had become interested in religion and "a cause". Asked what he meant by a cause he replied: "The freedom of Muslim lands from occupation.
"I still believe in that cause."
It is alleged the men plotted between 1 January 2001 and 31 March 2004 to set off a series of bombs.
Khyam, his younger brother Shujah-Ud-Din Mahmood, 18, and Waheed Mahmood, 33, from Crawley, West Sussex, along with Jawad Akbar, 22, from Crawley and Uxbridge, Anthony Garcia, 27, from Ilford, east London, Nabeel Hussain, 20, from Horley, Surrey, and Salahuddin Amin, 30, from Luton, Bedfordshire, are accused of conspiring to cause an explosion likely to endanger life contrary to section 3 (1)(a) of the Explosive Substances Act 1883.
Khyam, who has also lived in Slough, Garcia and Hussain are also charged under the Terrorism Act 2000 of possessing an article for terrorism - namely 600kg of ammonium nitrate fertiliser between 5 November, 2003 and 31 March, 2004.
The brothers Khyam and Mahmood also deny having aluminium powder, which is an ingredient in explosives, between the said dates.
Sick son of a bitch