Related story: American fighter jets secure the skies above NATO summit

BUCHAREST, Romania — In recent months, the tone has been tough. The words of warning have been fraught with potential ramifications for NATO operations in Afghanistan, where alliance in-fighting about troop level commitments has at least one nation threatening withdraw.

Canada, carrying a heavy burden in the volatile south, has cautioned that it needs more support from others in the alliance to stay in the fight. And in February, Defense Secretary Robert Gates offered his own critique, saying there are those “willing to fight and die to protect people’s security, and others who are not.”

This week, leaders from 26 NATO states are gathered in Romania, where they will attempt to bridge the divide on Afghanistan and confront an array of other hot-button, conflict-riddled issues.

In a pre-summit speech Wednesday morning in Bucharest, President Bush called on other allied nations to step up their efforts against terrorism. He insisted that nations “need to take this mission seriously,” saying it’s worth it for security and peace. The mission is too important to turn away from, and that failure could produce a safe haven for terrorists.

“If we do not defeat the terrorists in Afghanistan, we will face them on our own soil,” Bush said. “Innocent civilians in Europe and North America will pay the price.”

During his opening keynote speech Wednesday, NATO Secretary-General Jaap de Hoop Scheffer said he expects that the alliance will adopt a “vision statement” that will clarify NATO’s commitment to Afghanistan and pave a path forward.

But the conference has plenty of other business to tend to, he said.

Discussions at the summit will center on missile defense, Russian opposition to a proposed NATO expansion to the east, and operations in Afghanistan. State leaders were expected to delve into those issues during a closed-door dinner. The official working meetings commence Thursday morning.

During the Bucharest summit, de Hoop Scheffer said he has high hopes that more than one nation will be extended an invitation to join the NATO alliance.

“I’m hoping this will be a significant enlargement,” he said, referring to the proposed membership Croatia, Albania and Macedonia.

On Thursday, the issue will be decided.

NATO also will decide this week whether Ukraine and Georgia should be offered the prospect of joining NATO, though resistance is strong to such a move. Russia fiercely opposes an expansion into its old Soviet-era sphere of influence. And other NATO states, including France and Germany, have expressed reluctance about bringing Ukraine and Georgia aboard.

However, Bush pressed the issue, saying the alliance should be open to all European democracies. For now, that means the former Soviet republics of Ukraine and Georgia.

Bush said a larger NATO is not a threat to Russia.

“We must make clear that NATO welcomes the aspirations of Georgia and Ukraine for membership in NATO and offers them a clear path forward toward that goal,” Bush said. “NATO should welcome Georgia and Ukraine into the Membership Action Plan.”

Bush also said NATO members need to boost troop contributions to Afghanistan, where there are fears of Taliban and al-Qaida resurgence.

“The terrorist threat is real, it is deadly, and defeating this enemy must be the top priority of the NATO alliance,” Bush said.

The U.S. is the biggest contributor of troops in Afghanistan, with 17,000 in the NATO-led force and 14,000 in a U.S.-led contingent in eastern Afghanistan that trains Afghan forces and hunts al-Qaida fighters. The U.S. presence is set to go up by 3,500 Marines this spring, most of them dedicated to the NATO mission.

During a news briefing, NATO spokesman James Appathurai acknowledged that discussion about troop levels is unavoidable. “Canada has made very clear it requires a partner in Kandahar,” he said.

But whether the summit will result in the levels of support sought by Bush, Gates and military commanders is another matter. Gates, speaking Tuesday before the start of the summit, said he expects the summit to produce some troop commitments.

“(But) I would be surprised if we saw commitments in Bucharest at a level that would fully meet all the requirements for combat troops and military and police trainers,” he said. “But we’ll just keep working at it.”

The Associated Press contributed to this report.