Mistakes in Iraq and Afghanistan must not overcome the "moral pulse" to encourage democracy, Foreign Secretary David Miliband has said.
The spread of freedom would come "not just in words, but in deeds", he added.
Mr Miliband said he understood doubts over current conflicts, but argued it was important that "divisions" did not "obscure our national interest".
In a speech, he also rejected criticism that western countries were indulging in "intellectual imperialism".
The foreign secretary's comments came in a speech to an audience in Oxford, entitled the Diplomatic Imperative.
He said: "I understand the doubts about Iraq and Afghanistan, and the deep concerns at the mistakes made.
"But my plea is that we do not let divisions over those conflicts obscure our national interest, never mind our moral impulse, in supporting movements for democracy.
"We must resist the arguments on both the left and the right to retreat into a world of realpolitik," he added.
"In fact, the goal of spreading democracy should be a great progressive project; the means need to combine soft and hard power.
"We should not let the genuine debate about the 'how' of foreign policy obscure the clarity about the 'what'."
Fostering democracy in the Middle East was "the best long-term defence against global terrorism and conflict", and all available means should be used in that cause, Mr Miliband said.
He highlighted the need to support popular democratic campaigns in states such as Burma, and called for more reform in China - where he is due to go in a fortnight.
"China's incorporation into the global economy has brought radical social change. Chinese society is more mobile, vocal and diverse than in the pre-reform period," Mr Miliband said.
"Arguably more people in China are freer today that they have been at any previous time in Chinese history.
"But people inside China and outside are rightly concerned about the next stages in political development."