George W Bush says the United States is planning a comprehensive and sustained assault against terrorism.
He told US troops to get ready to retaliate for the terrorist attacks.
The president added that Osama bin Laden is the prime suspect and said he cannot "hide from the United States"
"Make no mistake about it, underneath our tears is the strong determination of America to win this war and we will win it," he said.
Of bin Laden, Mr Bush said: "If he thinks he can hide from the United States and our allies he will be sorely mistaken."
The president warned the campaign will be a different kind of conflict against a different kind of enemy. He says the terrorist attack on the US was aimed at tearing the country apart but had only bonded people together.
In a national radio address he said the perpetrators will soon discover that "those who make war against the US have chosen their own destruction."
He talked of a "series of destructive actions" against terrorist organisation and said he would not settle for a token attack, but plans instead to launch a sweeping and sustained effective response.
He added: "I have much to ask of the American people. I ask for your patience. The conflict will not be short or easy. The course to victory may be long."
The president told the 50,000 US reserves being called into action: "Everybody who wears the uniform (should) get ready - the United States will do what it takes to win this war."
PA