Gen Dannatt has spoken out against troop numbers in Afghanistan
The former head of the Army has said he did not accept an advisory role for the Tories as part of a "long-term plot".
Gen Sir Richard Dannatt has been criticised for taking up a role with the Tories so soon after stepping down.
But in a lecture he said Afghanistan was "critical" and if he had waited another year it could be "too long".
He said what motivated him was getting "these issues right in the interests of the nation and... the armed forces and the safety of our citizens".
Tory leader David Cameron officially announced during his conference speech on Thursday that Gen Dannatt would be advising his defence team and "will join our benches in the House of Lords and if we win the election could serve in a future Conservative government".
But the news leaked out a day earlier and was criticised by some as being too soon.
While he was still in his post as head of the Army, Gen Dannatt repeatedly called for better equipment for British troops in Afghanistan and has complained that he was smeared by the government for speaking out.
Labour peer Lord Foulkes, who criticised the general at the time, questioned how far his links to the Conservative Party went back and said it had been "unprecedented" for a serving officer to speak out publicly.
Labour's former defence minister Doug Henderson told the BBC that stepping into a political role so quickly "blurs the line between political decisions and military decisions".
And Gen Lord Guthrie, an ex-chief of the defence staff, urged Gen Dannatt not to take the Conservative whip, telling the Independent newspaper: "If he is going to the House of Lords, it's best to be a crossbencher.
"I will give advice to anyone, Labour or Conservative, but I wouldn't want to be associated with any one political party."
Speaking in a lecture to the Windsor Leadership Trust on Thursday, Gen Dannatt said he did not know Mr Cameron very well, but had met him on a few occasions - including a couple of substantive discussions over the past 12 to 18 months.
He said Mr Cameron rang him a few weeks ago to make his job offer.
Gen Dannatt added: "He put it to me that he was concerned that his defence team - at a time when defence was really important, and Afghanistan was really critical - lacked expert understanding, and would I be prepared to advise his team?
"And, if the Conservatives win the election, would I be prepared to take a peerage and maybe join his ministerial team?"
He added: "As with all things in life, it leaked out a little bit early. But that's how I come to be going to do what I might be doing and I think, to be honest, it rather indicates that it wasn't a long term plot we've been hatching up for a long time."
Asked why he did not wait longer to start, he said what motivated him was getting "these issues right" in the interests of the nation and armed forces.
"The trouble is in turbulent times, time is not on your side, and the operation in Afghanistan is really critical, we've got to get it right," he said.
"The mission in Afghanistan is really critical, we will succeed, we must succeed, it's got to be properly resourced, and frankly if I'd waited an elegant year, that could be a little bit too long."
BBC defence correspondent Caroline Wyatt said some of his critics believed a non-political approach was vital on Afghanistan and there were worries at the Ministry of Defence about the impact a recently retired chief returning as a minister would have on the relationship between serving commanders and ministers.