Along with a stunning addition to its membership, President Bush is also urging NATO to change its mission as well, to fight the "true threat" the world faces today — terrorism.

Bush arrived in Prague late Tuesday for a NATO summit that will invite seven nations that were once part of the old Communist bloc to become part of the European, Canadian and U.S. military alliance. The prospective member nations are Lithuania, Latvia, Estonia, Romania, Slovenia, Slovakia, and Bulgaria.

The president told Czech television on Tuesday that NATO must also change its military strategy.

"NATO used to be a way to defend Europe from the Warsaw Pact. But the Warsaw Pact no longer exists. Russia is not an enemy. And we face new threats, and the new threats are global terror," he said.

The president hopes to use part of Thursday's summit meeting to talk about ways that the existing 19 NATO nations can join together to fight those new threats.

The United States has also been prodding NATO to establish a 21,000-soldier rapid reaction force that could be mobilized in as little as a few days for emergencies.

Proposed by U.S. Defense Secretary Donald H. Rumsfeld in September, the force would not take action in Iraq should Saddam Hussein defy a United Nations Security Council resolution to disarm.

The United States and Czech Republic were already cooperating on Tuesday, filling the skies with fighter jets — including 15 American F-16's — to protect the summit even as security officials discovered a small explosive device under a railroad track in the city, but well away from the NATO meetings.

The Czech government mobilized 12,000 police officers, 2,200 heavily armed soldiers and special anti-terrorist units to protect the presidents and prime ministers attending the meeting.

Threats surrounding a Bush speech to students on Wednesday about the planned NATO force were so serious that it was moved from Radio Free Europe's headquarters to a sequestered hotel along the riverfront, law enforcement officials said.

"Terrorist attacks can happen wherever and whenever," Czech President Vaclav Havel said. "Our police and security forces have prepared a wide network of measures and have done the maximum so that nothing like that would happen. But 100 percent certainty cannot be found in the world today."

Bush argues that part of the war against terrorism includes disarming Saddam, and he is clearly looking to his NATO brethren — including Canada, which has consistently opposed military action against Iraq — in case push comes to shove.

"The U.N. Security Council has spoken, and says he must disarm. So it's his choice to make. If he refuses to disarm, then we will lead a coalition of the willing and disarm him. And of course, I hope our NATO friends come with us," he said.

Summit organizers have already suggested that NATO will not collectively fight Saddam, a request that Bush has not sought. They could find individual ways to support a campaign against Iraq, however.

For example, Czech army units specialize in dealing with poison gas, radiation or biological attack. Hungarian army engineers and guards earned respect on NATO duty in Bosnia and Kosovo.

As for help in a possible war with Iraq, White House press secretary Ari Fleischer said, "certainly there are many nations who see things as the president does."

NATO diplomats could put together a statement calling on Iraq to allow unfettered weapons inspections, but German officials said that to get Germany to sign on, the statement must not suggest any more "serious consequences" than stated in the U.N. Security Council resolution. The U.N. resolution does not get specific.

The president, who decided not to go back to the Security Council over Iraqi attacks on U.S. and British planes in the no-fly zone, seemed to be keeping his powder dry, with White House officials saying they ask for nothing more and that they also want to give weapons inspectors a chance to see if Iraq will cooperate with them or not:

"But we're not close to that decision point yet because we're just beginning the process of allowing Saddam the chance to show the world whether or not he will disarm," he said.

As Bush made his way here, a senior adviser to Saddam said in Baghdad that Iraq will meet a Dec. 8 deadline for declaring whether it still holds any weapons of mass destruction. Wary Bush advisers said they would believe that when they saw it.

Fox News' Jim Angle and The Associated Press contributed to this report.