Tony Blair told George W Bush he wanted to act fast against countries that possessed weapons of mass destruction (WMD) – and "explain and justify our actions" afterwards.
A private memo, entitled "Note for the President" was written by Blair the day after the 11 September 2001 terrorist attacks on the US. It was released on Wednesday as part of the UK's official inquiry into the Iraq war.
In the memo Blair set out his commitment to "action that some will baulk at" in order to intervene before countries provided weapons of mass destruction to terrorists.
He wrote: "[A]fter reflection, there will be many who ask: what is the next stage of this evil? What of their capacity to get hold of biological, chemical and other WMD? We know that there are countries and individuals trading in WMD and/or trying to acquire them. We need a range of sanctions and pressures to stop this.
"Some of this will require action that some will baulk at. But we are better to act now and explain and justify our actions than let the day be put off until some further, perhaps even worse catastrophe occurs. And I believe this is a real possibility."
Blair's memo was one of the many documents included in the Iraq Inquiry, led by Sir John Chilcot.
According to the inquiry, Blair often preferred memos to phone calls so he could provide a clear message to US presidents.