President Bush's upcoming trip to Europe could be an opportunity for him to press U.S. allies to put pressure on Syria and Iran, following the two countries' announcement Wednesday that they would defend each other against external threats.

On Tuesday, the U.S. government recalled its ambassador from Damascus, one day after the assassination of former Lebanese Prime Minister Rafik Hariri (search).

The Lebanese opposition blames Syria and the Lebanese government for Hariri's death; Bush said Thursday he would let an international investigation decide who was responsible.

Bush added that the diplomatic rift "indicates that the relationship [with Syria] is not moving forward, that Syria's out of step with the progress being made in a greater Middle East, that democracy's on the move and this is a country that isn't moving with the democratic movement."

Speaking to reporters after nominating John Negroponte for the post of national intelligence director, Bush said the international community needed to talk "clearly" to Syria.

The world has to make sure, Bush said, that Iraqi Baathists aren't being harbored in Syria, that the country not give safe haven for terrorist groups and that it abide by U.N. Security Council Resolution 1559, passed Feb. 9, which calls on Damascus to remove its troops from Lebanon.

"These are very reasonable requests ... all aimed at making the world more peaceful," the president added. "I look forward to working with our European friends on my upcoming trip on how we can work together on making rational decisions."

Bush is flying to Brussels, Belgium, on Sunday for a three-night stay that coincides with NATO and European Union meetings.

He will have dinner with French President Jacques Chirac (search), meet with NATO leaders and visit the headquarters of the European Union. The president afterward will stop in Germany for talks with Chancellor Gerhard Schroeder (search) and will meet with Russian President Vladimir Putin (search) in Slovakia.

Amb. Robert Kimmitt, former undersecretary of state under President George H.W. Bush, said it was important for Bush to have direct discussions with European leaders about Iran, Syria "and any other nations that are supporting terrorism elsewhere."

"This face-to-face dialogue is something we've been missing for a while. It's a great first step for this president," he commented. "[Iran and Syria] have been an 'axis of instability' in that part of the world for some time."

Some military and foreign policy experts say Syria has been such a source of instability that any drastic move by Damascus could rock the region and that U.S. military action should not be ruled out.

Frank Gaffney, an assistant secretary of defense in the Reagan administration, called Iran and Syria "the very essence of evil in a very turbulent part of the world."

Diplomatic pressure on both should come first, Gaffney said, but "paramilitary warfare" might be needed.

Aaron Miller, president of the U.S.-based Middle Eastern conflict-resolution organization Seeds of Peace (search), said that any action taken against Syria should be multilateral.

"Military action should be looked at, it seems to me, it should always be an option, it should never be taken off the table," Miller told FOX News. But "all of that has got to be last resort."

Bush's trip was planned long before the assassination of Hariri, a Sunni Muslim billionaire and politician credited with rebuilding post-civil war Beirut.

In recent months, Hariri had become a leader of the anti-Syrian Lebanese opposition. He and 13 others were killed by a massive bomb placed along Beirut's seaside road Monday.

Up to 200,000 people marched in Hariri's funeral procession Wednesday, many shouting anti-Syrian slogans.

The pro-Syrian Lebanese government has implied Islamic fundamentalists or Israel were behind his death, and rejects the Hariri family's call for an international investigation.

The U.N. Security Council has demanded that Lebanon find those responsible. Last September, the body passed a resolution telling Syria to get its approximately 15,000 troops out of Lebanon.

Whoever was responsible, the assassination put the issue of Lebanese sovereignty in the international arena.

"We support the international investigation that will be going on to determine the killers of Mr. Hariri," Bush said Thursday.

The United States is working closely with Chirac, a personal friend of Hariri.

A response to Syria is "rapidly climbing up the agenda" of the meeting next Monday between Bush and the French president, a senior administration official told The Washington Post. "We're on the same page. ... The entire international community wants to see something happen."

On Iran, Bush said the international community needed to "continue to speak with one voice — and that is, Iran should not have a nuclear weapon and how do we work together to make sure they don't."

The Military Option

Since the United States was accused of unilateralism in invading Iraq, experts say getting broad international support for any action — military or not — against Syria would a good way to help mend fences.

The White House can "begin to work with our allies to begin making an alliance that makes it clear to Syria that they have to move in Lebanon in a way that reduces their presence there," said former Rep. Bob Walker, R-Pa. "Here's a place where, I think, our European allies can join us in the War on Terrorism in a very positive way."

Secretary of State Condoleezza Rice, testifying before the Senate Appropriations Committee on Thursday, was asked by Sen. Robert Byrd, D-W.Va., if military action against Syria was in the works.

"Concerted international pressure of the international community can and should move the Syrians to act in way in accordance with the U.N. resolution," Rice responded. "We have many diplomatic tools at our disposal ... we are employing those tools."

The military option may be fraught with obstacles, including the availability of U.S. troops ready for duty.

"President Bush has a tough problem because we're spread very thin right now ... he has a lot on his hands," said Democratic strategist Michael Feldman, who added that congressional Democrats likely would support military action against Syria.

Added GOP strategist Matthew Dowd: "I think the American public is going to give him a tremendous amount of leeway and trust to deal with that."

Gen. Burton Moore, U.S. Central Command director during Operation Desert Storm, called the Syria-Iran partnership an "unholy alliance" that threatens not only Israel but the world.

"I think we can lay a marker down, and we can do it very quickly, very simply, relatively," Moore said. "The Bush administration has shown it can walk and chew gum at the same time ... we can send a real strong signal with one bomb from a B-2 to a Syrian-regime target."

Former Secretary of State Lawrence Eagleburger said "the Syrians have gotten away with murder for more than a couple of years and it has to be stopped."

"If in fact they are ... putting supplies across the border as they do with Iraq, then I don't see anything wrong with bombing along the border" with Lebanon, he continued.

But Eagleburger said it's unlikely the United States wants to do any bombing because the Europeans may not be able to stomach it.

"I certainly don't want, at this stage, to get everyone nattering at us again," he said.

After no weapons of mass destruction were found in Iraq — the main justification given by the administration for the invasion — others say it may be difficult to make the case for military action.

"I'm not really sure the president would be able to get the will of the American people to do something militarily with Syria," said Washington Times writer and FOX News contributor Bill Sammon.

Sammon added that Bush would be between a rock and a hard place if Russia and other European countries did not get behind sanctions first.

If diplomatic efforts against Syria fail, said former Florida Democratic Sen. Bob Graham, "we should lay the law down to the Syrians that they get rid of groups like Hezbollah or Hamas, or we get rid of them ... I don't think we need to wait for people to join us."