• Bush admits: Tony Blair was my closest foreign ally • Waterboarding 'helped to break up terror plots in UK' • Iraq was the right thing to do, says former president

George Bush ordered the Pentagon to plan an attack on Iran's nuclear facilities and considered a covert attack on Syria, the former president reveals in his memoirs.

Bush, in the 497-page Decision Points, a copy of which was obtained by the Guardian in advance of its publication in the US tomorrow, writes of Iran: "I directed the Pentagon to study what would be necessary for a strike." He adds: "This would be to stop the bomb clock, at least temporarily."

Such an attack would almost certainly have produced a conflagration in the Middle East that could have seen Iran retaliating by blocking oil supplies and unleashing militias and sympathisers in Iraq, Afghanistan and Lebanon.

Bush also discussed with his national security team either an air strike or a covert special forces raid on an alleged Syrian nuclear facility at the request of Israel.

The book, which is published in the US tomorrow, seeks to rebuild Bush's reputation, giving his side of the story on the most controversial issues of his presidency, which include Iraq, Afghanistan, hurricane Katrina, the Wall Street meltdown and torture at Guantánamo.

Bush justifies the use of waterboarding in his book, saying that the controversial interrogation technique used on three detainees helped break up terrorist plots to attack Heathrow airport, Canary Wharf, US diplomatic missions and a number of targets in the US. He writes: "Whatever the verdict on my presidency, I'm comfortable with the fact that I won't be around to hear it. That's a decision point only history will reach."

In the memoirs Bush:

• Describes Tony Blair as his closest foreign ally.

• Admits mistakes over Iraq, but regards it as the right thing to have done.

• Defends the Guantánamo Bay detention centre and the use of torture.

• Accepts he took "too long" to make decisions over the disaster that engulfed New Orleans after it was struck by hurricane Katrina five years ago, killing more than 1,800 people, but says the blame lies with other people.

In a book largely lacking in personal insight, Bush says he is most angry at accusations that he was indifferent to the plight of the victims of Katrina because so many are black. "The suggestion that I was a racist because of the response to Katrina represented an all-time low. I told Laura at the time that it was the worst moment of my presidency. I feel the same way today," Bush writes.

On Iran, some of his advisers argued that destroying "the regime's prized project" – its nuclear facility – would help the Iranian opposition, while others worried it would stir up Iranian nationalism against the US.

Two other options under consideration by Bush were direct US-Iranian negotiation, which Barack Obama favours but Bush ruled out, saying talking to a tyrant seldom worked out well for democracies; and joining the Europeans in a mixture of sanctions and talks with Iran, the option he finally chose.

"Military action would always be on the table, but it would be my last resort," he said. He added that he discussed all the options with Blair, who in his memoirs, published earlier this year, revealed he is now leaning towards military action.

Bush says: "One thing is certain. The United States should never allow Iran to threaten the world with a nuclear bomb."

Bush also discussed a request from the then Israeli prime minister, Ehud Olmert, to bomb a suspected Syrian nuclear plant. Bush convened his national security team to discuss an air strike or a covert raid. He says of the latter: "We studied the idea seriously, but the CIA and the military concluded it would be too risky to slip a team into and out of Syria." He said no to a disappointed Olmert. The Israelis then did it themselves in September 2007.

Bush's first call after 9/11 was with Blair. "The conversation helped cement the closest friendship I would form with any foreign leader," Bush writes. Blair is referred to at various points as "Tony", whereas the French president, Jacques Chirac, who kept France out of the Iraq war, is referred to simply as "Chirac".

Bush confirms that planning for an invasion of Iraq began within two months of 9/11 – but insists that war was not inevitable, even in the final weeks before the invasion. He is critical of John McCain, who unsuccessfully ran against him in 2000 for the Republican nomination and against Barack Obama for the presidency in 2008.

In the midst of the latter campaign, with the banks and the financial industry falling apart, Bush called Obama and McCain to the White House for an emergency meeting.

McCain, the senator for Arizona, was trailing in the polls, but Bush thought the financial crisis offered him a chance of a comeback. He could make the case that he was the better candidate for the times – experience and judgement over youth and charisma.

At the White House, Bush says he was "puzzled when McCain passed up the chance to speak".

In a separate interview to publicise the book, Bush traces his opposition to abortion back to his teenage years when his mother had a miscarriage, kept the foetus in a jar, and showed it to her son.

"She said to her teenage kid: here's the foetus," Bush told NBC. "There's no question that affected me, a philosophy that we should respect life … There was a human life, a little brother or sister."

Bush said that the purpose of telling the story "wasn't to try show the evolution of a pro-life point of view".

"It was really to show how my mom and I developed a relationship."