"Russia is not our enemy," said Mr Bush
Mr Bush said the missile threat was from the Middle East, not Russia, which strongly opposes sites for the shield in Poland and the Czech Republic.
He warned that Iran could have a ballistic missile capable of reaching Europe or the US by 2015.
Earlier, US Defence Secretary Robert Gates said the shield could be delayed while Russian concerns were tackled.
Iran threat
In a speech at the National Defence University in Washington, Mr Bush said: "The need for missile defence in Europe is real and I believe it's urgent."
The mayor of nearby Slubsk is disappointed that the base may be put to this use [US missile defence], rather than turned into a civilian airport and business centre
Mark Mardell
BBC Europe editor
Euroblog: Poland's fears
He said the planned system was not designed to tackle missiles from Russia as it would be easily overwhelmed by Moscow's arsenal.
"The Cold War is over. Russia is not our enemy," he said.
Mr Bush said the US had invited Russia to "join us against an emerging threat that affects us all... we ought to respond to this threat together".
The president said if "rogue states" had less confidence their missiles would strike, they would be "less likely to engage in acts of aggression in the first place".
Mr Bush also attacked the US Congress for reducing funding to missile shield systems.
Earlier, Mr Gates had said activation of the European shield could be delayed until there was "definitive proof" of a missile threat from states such as Iran.
He said after meeting Czech Prime Minister Mirek Topolanek in Prague: "We would consider tying together activation of the sites in Poland and the Czech Republic with definitive proof of the threat - in other words, Iranian missile testing and so on."
The missile shield system would see a radar site set up in the Czech Republic and a missile interceptor base in Poland.
Russia has vehemently opposed bases on the territories of its former Warsaw Pact allies.
Mr Gates and US Secretary of State Condoleezza Rice received a frosty reception when they tried to sell the plan in Moscow this month.
Russian Foreign Minister Sergei Lavrov said Moscow saw the shield as a "potential threat" to its security and wanted to "neutralise" it.
Russian President Vladimir Putin widened the debate by also threatening to abandon a key nuclear missile treaty.
He said it would be difficult to remain part of the Intermediate Range Nuclear Forces (INF) treaty unless it was expanded to include more countries than just the US and Russia.
George W. Bush speaking at NDU on October 23, 2007.
In a speech on Tuesday, U.S. President George W. Bush said that deploying a missile shield in Europe is necessary to counter an emerging nuclear threat from Iran.  The planned missile shield is strongly opposed by Russia, which sees it as a threat to its security.

"The need for missile defense in Europe is real and I believe it's urgent. Iran is pursuing the technology that could be used to produce nuclear weapons, and ballistic missiles of increasing range that could deliver them," Bush said in a speech at the National Defense University. "Today, we have no way to defend Europe against the emerging Iranian threat, so we must deploy a missile defense system there that can."

In his speech, Bush emphasized the threat posed by the range of Iran's missiles. "Last November, Iran conducted military exercises in which it launched ballistic missiles capable of striking Israel and Turkey," Bush said.  He warned that, with "continued foreign assistance", Iran could develop an ICBM capable of reaching the United States by 2015.

Russian President Vladimir Putin has suggested that the United States and the West could rely on Russian-operated early warning radar in Azerbaijan to counter missile threats from Iran.

The U.S. missile defense plan includes 10 ground-based interceptors in Poland and a radar system in the Czech Republic. Russian Foreign Minister Sergey Lavrov said previously that the shield was seen a "potential threat" by Russia and that Russia could take measures to "neutralize" it. In his speech, Bush said that the missile shield was not designed to intercept missiles from Russia and "would be easily overwhelmed by Russia's nuclear arsenal."

Also on Tuesday, U.S. Secretary of Defense Robert Gates proposed delaying the activation of part of the missile shield if Russia cooperates with the project.  "We continue to encourage the Russians to partner with us in missile defense and continue our efforts to reassure them that these facilities are not aimed at Russia and could benefit Russia," Gates said. He also suggested that the missile shield could remain inactive until "definitive proof" of a threat arose. "We would consider tying together the activation of the sites in Poland and the Czech Republic with definitive proof of the threat, in other words, Iranian missile testing and so on," he said.